I have no hard-and-set opinion on this question, and naturally tend to err on the side of caution. However, I do have a few thoughts that I'll air here.
The first thing I'll point out is that whether deaconesses are the Sacrament of Holy Orders or just a sacramental comparable, perhaps, to minor orders or something like that…is really an abstract debate.
Given that deacons have no “new” sacramental powers that can't also be delegated to the laity (baptisms and witnessing marriage)…this question will never affect the validity of any Sacraments, so it is of no practical effect.
So someone claiming it will never happen is, in some sense, making a completely non-falsifiable declaration, because it is merely equivalent to saying “No woman will ever receive the indelible character of the Sacrament of Holy Orders in any of its three grades” which is an invisible reality.
It doesn’t mean, however, that a ceremony (possibly even involving a laying on of hands) won't ever be approved (with similar language to a deacon’s ordination) to create something called a “deaconess” again. It just means that the speaker is of the opinion that this would be a sacramental rather than the Sacrament of Holy Orders.
But, as the Catholic Encyclopedia article on sacramentals says, they “are named sacramentals because of the resemblance between their rites and those of the sacraments properly so-called.”
So “it will never happen” is a non-falsifiable claim. Because even if something “looking like” an ordination of deaconesses occurred, supporters of this opinion could just say, “Yes, but that’s just a sacramental resembling Holy Orders, like the minor orders, but not the Sacrament proper; just as it also was in the early church.”
Don’t expect most lay people, however, (nor the Eastern churches) to grant much importance to this theoretical theo-ontological distinction, however! Especially when there wouldn’t have to be any difference practically speaking.
This, of course, could lead to a slippery slope (pushing for priestesses, etc) that could invalidate Sacraments. This is why I tend to think, whether they are the Sacrament or merely a sacramental, any sort of restored deaconesses should likely be limited to monasteries of cloistered nuns.
As for the theoretical question itself, I think the essential unity of the three grades of Holy Orders is the strongest argument against deaconesses being the Sacrament proper. However, I had a thought the other day that it might be possible to argue that the exclusion of females from the higher two grades of Orders might be "accidental" to the essence of the Sacrament itself. Which is to say, it could be argued that the essential unity between the three grades of Orders is simple the character. On the other hand, the fact that priests can celebrate Mass and absolve and anoint and confirm and that bishops can ordain and, also, have the magisterial office...is something that is different between the three grades (since deacons can't do any of that, and bishops have roles simple presbyters don't) and therefore can't be called part of their essential unity.
Therefore, though we know that females are incompatible with the higher two grades for sure, the question of whether they are incompatible with the lowest grade depends on why they are incompatible with the higher grades. If it is because the female sex is incompatible with the essential character itself, then the unity of the Sacrament would exclude them from the diaconate. If, however, what excludes them is simply that they are incompatible with the adjunct features of Sacramental powers, especially of acting in persona Christi at the eucharistic consecration...then female deacons would not necessarily threaten the essential unity of the Sacrament anymore than the fact that priests can celebrate Mass even though deacons can't, or that bishops can ordain even though the lower two grades can't. Under such an argument, women would not be considered ontologically incompatible with Holy Orders in itself (as souls are not male and female) but nevertheless would be necessarily invalid recipients of the higher two grades for reasons of something (acting in persona Christi) that is necessarily and intrinsically adjunct to those grades of the character.
Therefore, though we know that females are incompatible with the higher two grades for sure, the question of whether they are incompatible with the lowest grade depends on why they are incompatible with the higher grades. If it is because the female sex is incompatible with the essential character itself, then the unity of the Sacrament would exclude them from the diaconate. If, however, what excludes them is simply that they are incompatible with the adjunct features of Sacramental powers, especially of acting in persona Christi at the eucharistic consecration...then female deacons would not necessarily threaten the essential unity of the Sacrament anymore than the fact that priests can celebrate Mass even though deacons can't, or that bishops can ordain even though the lower two grades can't. Under such an argument, women would not be considered ontologically incompatible with Holy Orders in itself (as souls are not male and female) but nevertheless would be necessarily invalid recipients of the higher two grades for reasons of something (acting in persona Christi) that is necessarily and intrinsically adjunct to those grades of the character.
Either way, I think the very fact that the topic is being discussed so much lately is interesting and means something is afoot. Those pushing for women priests have been vocal for decades, but do seem pretty much marginalized at this point, at least in terms of influence over the institutional Church. But I think it is indicative of some sort of "moment" in the Church that this more moderate proposal is now being floated, and that even conservatives like Ed Peters are admitting that "Ordinatio says nothing, however [...] about ordaining women to diaconate nor, strictly speaking, does it address (at least not definitively) ontological questions about female ordination. In that regard, discussion may continue" even while personally being disinclined to the idea.
What will come of this moment remains to be seen. Perhaps a firm assertion that it is impossible. Perhaps a declaration that it is. Or perhaps a restoration in cloisters of deaconesses, with a corresponding declaration that "Whether this is a Sacrament or a sacramental is a matter for speculative theology, as it has no practical effect either way." Or perhaps just greater administrative roles for women even while reserving Sacramental or liturgical roles to males. Who knows.
What will come of this moment remains to be seen. Perhaps a firm assertion that it is impossible. Perhaps a declaration that it is. Or perhaps a restoration in cloisters of deaconesses, with a corresponding declaration that "Whether this is a Sacrament or a sacramental is a matter for speculative theology, as it has no practical effect either way." Or perhaps just greater administrative roles for women even while reserving Sacramental or liturgical roles to males. Who knows.