Not really meaning to necropost here, but our group has managed to try: 5e for two campaigns in prebuilt adventures, one campaign in PF2e using a prebuilt adventure, two homebrew campaigns in PF2e, all five of those with the same GM, each has run for approximately a year. Second player who GMs has run a Savage Worlds game with a my hero academia-inspired homebrew setting using supers rules, and is now running a PF1e homebrew mythic campaign. He GMs less often, only running for half a year so far on both games. Both of those GMs run once a week, one runs saturdays, the other runs sundays. About half of both groups go to both games, and half of each is only active in one game.
I give the first GM one day off each month by running no-prep dungeons using WOiN RPG on the first saturday of the month, using tools from Dungeon World to run the adventures no-prep. This group has also shown interest in playing Ars Magica, run by me, which will be starting after a while.
I've found the most effective way to get a group together with a new system in our group is to simply say "I'm running X, who wants to play?" and we are able to get a good group together. Each migration to a new system we've lost a player or two, but the friend group brings in more people as friends of friends, and the size of the group has kept pretty constant. The Saturday games have about 6 players and a GM, and the Sunday games have 8 players and a GM.
I know for some groups this wouldn't be much of an option, not everybody has a big enough social circle to keep bringing people in, but it works for our group, and I think that it might work for more than you'd expect. We've gotten a number of people who never did RPGs before into the games we play this way.
