The decision is obviously already made, but I don't think the numbers argument is really a valid one. When I wanted to play on GTA:W, I downloaded Rage MP. I wasn't just blindly browsing through the server list of an MP platform that I'd never heard of — I downloaded it because the community was attractive. And at the end of the day, it's all on GTA V, so it's not like the barrier of entry is exactly a big one once you've overcome the hurdle of getting the game. People literally just have to download a different platform, there is very little change for the end user.
FiveM also wasn't big in the beginning, which I know because I played on FiveM servers before there were any with more than 32 slots. What made FiveM big was good communities with impressive features (NoPixel, DOJRP, Twitch RP, Family RP, etc.), it wasn't the other way around.