I can agree with this. The potential that Void Elves could have is one of the things I like about them and something I hope actually gets explored. It'd be a terrible shame if they were just left as they are by the wayside.
Alleria did not "get caught up" in a "transformation process" and does not constitute foreshadowing of Void Elves IMO. Alleria had void powers long before her encounter with L'ura. Alleria just gained the Entropic Embrace void form for the first time after consuming L'ura's power.Alleria's transformation is foreshadowing that other elves can be caught up in that transformation process too.
Umbric's group, on the other hand, were caught up in a process that was intended to turn them into Void Ethereals. That process was interrupted in the midst of the ritual and resulted in Umbric and his group being markedly different from Alleria, further distancing her from them as far as origin goes. Do they all qualify as Ren'dorei (children of the void)? I'd say so. But make no mistake, Alleria is not a Void Elf in the same way that Umbric is a Void Elf. Alleria did not undergo a transformation process, at least not in any way related to what happened to Umbric and his group. So to say her story was foreshadowing for Void Elves is a stretch to say the least.
Alleria acquired power over the void before the Argus story. Her encounter with L'ura merely increased that power. Umbric and his crew were forcibly transformed into something new by the interruption of a process that was never intended to turn them into what they currently are. How new Void Elves are coming into their power has yet to be answered and is part of what makes Void Elves feel lacking in foundational background. We lack lore defining what they are, how they are coming into their power, and why new elves are seeking them out to join them.
We can infer things by the presence of the Scholar and Wayfarer NPC's, and now the new customization options, but without any lore added by the devs, whether through in-game additions or outside-of-the-game media, we are left with only head canon conjecture. That makes Void Elves feel unfinished compared to other allied races and thus the "asspull" label is applied to them.