Yo uhh
As someone who tried to get into Code of Princess and Kid Icarus Uprising competitively, having 30-500 players show up at a tournament is waaaaaaaaaaaaaay better than having the entire scene in your country be like... 8-100 players who live nowhere near each other.
Like with CoP, there were probably 8 of us who were trying to actually get good/loved the game, even tho it was broken, and a total of like... 24 people who played. Playing the same person in the same MU (or even different MUs) every single day gets kinda boring, especially when meeting new people and making friends with people you -never- would have met otherwise. Our Melee scene was the same way for -years-. Anybody outside of our group who showed up from inside the state would just get straight bodied. The only reason I think it didn't die out like CoP is because things like DBR's combo videos kept coming out, and we got videos off DC++ that showed us we weren't even -close- to what was possible in the game (SuperDoodleMan, iirc.) That motivated us to keep getting better. When you're literally one of the top 4 players in a game with like... 32 total players, it's -really- hard to stay motivated.
You say you'd play LoL if it had a tiny scene, but if you only had like... 100 people playing the game, no updates, none of the hype, I sincerely doubt you would. Maybe you would, but there's something really nice about meeting new people constantly and knowing you can go to almost any major city and find a scene that's completely unique. It's hardly shallow to say "hey, how big is this scene?" Our GG scene was -tiny-, and I was one of like... 5 US Zappa players at the time. We devoured Japanese Zappa players' videos, but being able to just hop online at any given moment and talk to a large pool of people who love the **** outta the character and are trying to up their meta constantly is -really- nice. For Zappa players in the US, it was rough. He's still my favorite character in any fighter to date, butyeah...
If it weren't for online play, I wouldn't be playing s4. I'm currently stuck in a town that's tiny, and even if I got lucky and found someone to play with (not happening,) unless they came from a similar background, I'd just absolutely body them. And I'm terrible at s4. There'd be no reason to play, because it would just be me destroying them and picking up a ton of bad habits along the way while trying to nurture them.
But with online play, I can find people regularly. Even FG is better than a game with no scene. I can pick up matches whenever I want, instead of having to match schedules with someone just to practice pvp. As is, my improvement is significantly hindered by the fact that I don't have someone to just sit there with a controller and do the exact same moves to me for 20 minutes straight so I can figure out what to do about them. If I lived in a city with a large scene, I could find that at 2am, IRL, and we'd probably get food and get to know one another.
Soyeah. It's a legit question.
And to answer OP's question: Smash 4 is big enough for some people to make a living off it. Not many of them, but it -is- possible. Not saying you'll be one of them (no offense, maybe you will!) Just that it's really, really hard to be that good and that consistent. But if a scene is big enough for pro players to exist, then it's definitely big enough to look into. Idk about compared to melee, but hey, less chance of carpal tunnel, so that's nice! And you don't have to practice tech for 30min/day to stay near the top. Hell, right now most the top players don't even know how to DI. :s