While those factors do not help, it's worth remembering that Smash as a multiplayer game doesn't risk becoming "stale" the exact same way a single player game does. Multiplayer games are heavily based around the notion that people by nature are going to play differently from each other - this is not only reflected in character choices, but also how they play the characters.
Take
for example. A lot of people (especially online) play him like a zoner - tossing Crowns and Cannonballs in every single situation no matter what. But thanks to his Belly Armor (and the related Crownerang armor) players can also choose to get into the fray with him. Sure, he has a ton of endlag, but he hits hard and rewards conditioning opponents, which also encourages people to play kinda like K Rool. As a big heavy boss.
Characters that lack projectiles are not exempt. While I'm not a fan of
's design due to it being railroad-y, he can still choose to either go for an offensive playstyle thanks to his ability to straight up win the ground game, but can also play more conservatively and react to more specific circumstances. In
his defense of Mac (that I only just recently discovered),
Thinkaman wrote this:
While Mac's intentionally built to suffer in the air, he's not entirely helpless there - which grants players just enough freedom to go for Jolt Haymakers off stage or even D-Air if one's feeling extra. But it also allows Mac players to play more reactionary if they want to since they can jump over Link's projectiles with a well-timed Side B.
This is also something that MOBAs have, despite MOBA characters typically having only 4 spells (+ additional couple spells / activated items). League players suddenly notice that Ezreal can either build standard damage dealer or he can also build tank items in order to kite people? Dota players noticing that the hard support Io (the Wisp) - based around healing, buffing and teleporting the ally the Wisp's linked to - has had certain elements buffed to the point the Wisp can also build hard carry with an endlessly respawning set of exploding Spirits?
Now that's the kind of stuff that keeps people sticking around. It's no wonder that League's been going for 12+ years, and Dota for 17+. Other examples exist in other genres like WoW, StarCraft and even Street Fighter II. What helps Smash's longevity is that at the end of the day you can decide to play a match with all kinds of wacky settings and however a person wants to. And learn wacky stuff about what the characters do - this is how people discovered Peach can pull a Bomb outta nowhere, after all.
Multiplayer games usually become seen as stale when a certain aspect comes to dominate how people play for extended periods of time or a game loses much of its luster. "Seen it all", in other words. Considering that casual players have brought out Smash consistently over the years for fun parties and that more competitively inclined people have continued to partake in tournaments over the years, it's safe to say that Smash's not getting stale any time soon for a lot of people.
Oh and as for single player games? Single player games can also have a lot of replay value - whether it's through silly glitches ("YAHOOYAHOOYAYAYAYAYAYA..."
zooms away to a parralel universe), dicovering new stuff like a room dedicated to a Nintendo Power subscriber or just because one wants to revisit a game. Single player games usually become seen as "stale" when there's not much else to do. While it's also "seen it all" it differs a bit from multiplayer games - single player games typically have an end goal, multiplayer games not neccessarily so.
And sometimes people just want to hit you with K. Rool's belly. not knowing or caring one bit about how much minus on shield that Dash Attack is or if Belly Armor's about to blow up in their face.
(That said though, yeah I do agree that a stronger online and more single player modes would help Smash immensely.)