I don't work in the sales department of the video game industry, so it's likely i'm missing something, but from my perspective there's a weird relationship between mechanics and marketability that seems almost entirely contrived. i think if tf2 came out a couple of years ago in the state it's in now, it would've been huge. there are a lot of things developers can add to games to make them more appealing to a casual audience. for example, fortnite actually has quite a lot of mechanical depth in its building/editing and the speed at which you're required to do it, but the average 12 year old only really cares that they can run around in a chicken costume and shoot grenades. and even in the fortnite world, where you're playing a battle royale with hardly any (if any at all) skill-based matchmaking algorithms, these casual players are sometimes getting smacked around by players like tfue without even realizing it. and they're still logging in to play everyday.
a game like melee had an incredible underground scene that learned all of the crazy techniques you see people still mastering and utilizing 2 decades later, but from a casual standpoint the game was a massive success. for some reason people/nintendo seem to think otherwise? i'm pretty sure the sales for melee were monumental, and there was no need to make brawl a game that tried its hardest to eradicate the competitive side of the franchise.
i would imagine that if you're really new and have a casual approach to games, most of the mechanical depth in melee, csgo, quake, tf2, etc. will go entirely over your head and you won't even have to experience much of it, because you'll play with and get matched against people in that same bracket. it seems like the key in the development of mechanically deep games is to have a very friendly and shallow base to your game that everyone can enjoy for what it is, while supplementing that with cosmetics and extra content.