You..............are..................wrong. At least as far as your general statement is concerned. This is not a zero sum thing. Some few people would be dissatisfied. Most, though, who like two or all three classes, would still be satisfied, just not quite as satisfied as if they received the sorcerer.
Exactly. Most would be satisfied to get one.
A LOT of complaining by a minority of people, yes. Just like has been happening since 5e came out with, let's see............barbarians, bards, clerics, druids, fighters, monks, paladins, rangers, rogues, sorcerers, warlocks and wizards. All classes have received a lot of complaints. Some more than others, but no class has been exempt. There's no reason to refuse to make a psion just because some people will complain.