You are talking at two different levels. When you talk about low skill level gameplay tiers don't even look the same.
Leroy isn't actually even that good at low levels play and on the reverse characters like Bears become S tier.
When you want to discuss tiers you have to look at high levels of play, and at high levels of play tiers matter a lot.
I don't play other fighting games so I wouldnt know about their balance.

"Easy to use" characters aren't often actually easy to use at low levels of play.
Kazumi isn't a strong pick in the hands of a bad player, even tho she is suuuper easy.
The reason for that is that she is easy if you have the required amount of tekken fundamentals.
But if you dont carry those fundamentals, Kazumi doesnt have anything she can do to edge out wins.

Tier lists change depending on the level of play, at lower levels, cheese, frame traps and strong offensive flowcharts reign supreme.
At higher levels of play, spammable neutral solvers like steve df2, good counterhit tools and strong pokes and lows become more impactful.

Also on the topic of "reaching skill ceiling", nobody is reaching skill ceiling or getting even close to it in tekken any time soon or possibly ever, the game is just too complex for that to ever happen.