Keep in mind that overly complex character generation and-or advancement can act as a discouragement to players, without whom there's no game.
It's linked to complexity at the table, though - chances are very good that a character that is highly complex and fiddly to generate is going to be highly complex and fiddly to play.
Creation complexity and table complexity can be related, but that doesn't mean they are the same.
Let me take 5e for example. Picture the complexity when you first started playing/running -- before you had built up a working knowledge of the classes and spells.
Now consider adding just the character portions of XGtE (chapters 1 & 3) to the PHB. You've just increased character creation/leveling complexity a good deal. There are a lot more choices for subclasses, a load mroe intereactions if you allow multiclassing, a bunch of new spells.
You've definitely increased character creation/advancement complexity, say 30% to pull out a number. But did you have a linear increase in complexity at the table? No. You may have had a slight increase as there's less overlap in spells or something, but your complexity for within a session is pretty flat.
Amber Diceless had a light mechanical table complexity, while it had a more involved character creation process that included as part of it several bidding wars between players.
Take the "rating 10" HERO system. It's character creation and advancement are extremely open-ended and math-tastic. That fully deserved to be the heavy-weight champion of character creation. But at the table, it's maybe a 8. There's a limited number of options, they are spelled out. Combat (as a proxy for mechanical complexity) takes a long time, but part of that is the many-rounds nature of the combat in order to represent the genre. I'm not saying it's not heavy, but the table complexity rating is not the same weight as the creation/advancement complexity.
And to take a step further, HERO has been a staple in the Supers genre for so long through so many editions and expansions in part because of how all-inclusive the creation system is. No system is for everyone and this isn't an exception, but continued commercial success I think shows that creation complexity need not be a turn off for players.
Again, that's a big call on the system. Twilight 2000 had a complex character creation that didn't have the payoff for that level of complexity -- there it easily could drive players off like you said.
So an appropriate level of character complexity to support mechanically the genre seems to be the sweet spot, even if that is higher in complexity. And while character complexity often informs table complexity, I feel it's a mistake to assume that they move in lockstep.