I'm not usually the biggest fan of crossovers either, which I think is a testament to how good Smash is. It's become an integral part of my life because for years it has continued to be fun, deep, interesting, and still surprises me occasionally with new possibilities and stuff I never even thought about. It's as good as a competitive game as anything else out there, if not better - what makes it so exciting in comparison to other fighting games in my opinion is the sheer amount of possibilities. Whereas in other fighting games you only have a health bar and you lose when it's depleted, in Smash there are many more factors at play with the % system, which on its own already makes it hard to lose a game by a safe poke (something which happens far too often in other fighting games). Instead, finishing an opponent off usually means you had to land a strong hit, or do something a little crazy. But furthermore, because of things like DI, SDI, the % system itself, different character weights and gravity and more, it means it's not too often you see a certain combo or a certain move always have the same effect. Whereas in other fighting games, when a combo starts, it's usually only down to the attacking player to get the inputs right, in Smash it's also down to the defending player to try to escape with good DI, which can often save you out of a situation where you couldn't do anything otherwise. It's just a far deeper game in my opinion, thanks to the core fundamentals of it rather than steep technical difficulty curves.
Also OP, when you talk about balancing "making sense", I hope you don't mean in a canon way. With Smash Ultimate coming out featuring potential matchups such as Dark Samus vs. Pichu, I think it's quite obvious the balancing won't "make sense". However, the games have gotten progressively better at balancing, despite the ever-increasingly-huge roster. Smash 4 could be a prime example of amazing game balancing, if only the latest DLC characters (Cloud and especially Bayonetta) didn't completely ruin it all. However, with Ultimate being built off Smash 4 as a foundation, and taking a more serious approach to competitive play, you can expect it to be pretty well-balanced.