1st paragraph: Again, irrelevant. The stack is 15k, you're 15k, so the match is balanced. If that's all they managed to get while 5-stacking then their lobby, OVERALL (AVERAGE), is 15k, even if some players are better or worse.
"A guy can get 15k rating soloq and then play a game in a stack".
Correct, this is the only instance where this is a problem. But this is rare and he won't last in his current elo, he'll move up and be hardstuck somewhere else, maybe 17k. Maybe do the same and find some people to play with and stop crying? Ever thought about that?