No thanks.
The game just has less goals if we do that. Why should someone who grinds out the HUGE amount of dust to max multiple legendaries be not allowed to do so? And why do you care? They'll tear down your gym with trash now a days they'll just do it faster with good mons. Plus they'll give those less fortunate better mons they don't deserve raid bosses with their good crews.
If you want to attack with only one legendary there is exactly nothing stopping you from making them rare and legendary for you. No reason to force others to do that though.
JMO.
