Kids these days...
No, VCAs are not about summing and summing engines.
VCAs are about control.
Film orchestra example:
Orchestra recording, when mixing I want to be able to set all the tracks individually, but at the same time balance 1st violins, 2nd violins, violas, cellos, contrabass and decca tree. And I may also need to control the 1st, 2nd and violas as a unit, and cellos and contras as another, and ALSO. Control the level of the recorded contra basses together with the overdubbed synth bass, but I obviously want to keep the actual sounds separate as they go to different stems but the control needs to be related.
Mixing it down to many groups is unecessary and limiting. I may need one strings stem and one synth stem so I don't want them mixed. But I DO need the control.
Bgvox, I want to be able to control the BGvox on one fader, but the processing may be widely different, and I do not want to limit myself artistically by mixing them down using a group. I may also want to control the level of the BGvox together with the synth vocoder so the balance stays the same, but I do not want to mix them.
I may want to control the level of all thrums, acoustic and electronic at the same time, but without mixing them.
And no, links and groups does not do the same thing. You may think it does, but it does not.
Maybe it's only old farts like me that have used real large scale hardware consoles that all had VCAs that fully appreciate the functionality, and if it is true that it is coming in the next gen release, I will be very happy.
I promise that when you truly understand VCAs, you do not want to mix without them (although you can, I have made it without them for a long time, but have missed them all the time).
VCAs is one of the few things I envy the implementation of in PT.