And so does a warlock portal. A monk can teleport with transcendence, a warrior can jump, a DH can double jump and fel rush or a DH tank can jump even further. They are all different just as a warlock portal is.
Your flawed comparison is like a 1% speed increase from a speed stat on your gear vs sprint from a druid. Just so wrong.
And who talks about class stacking? You already have multiple of the same classes in 20m due to their superior performance. Nobody suggested a 8 warrior 2 healer setup should beat CE prenerfs. You make weird assumptions and extremes to back your flawed logic.
Diversity is even better in 10m as you are more limited in who you bring. You simply can’t stack the top performers as much since you need other utility in your raid.
You clearly have never seriously done recruiting in a CE mythic guild for longer periods of time. It shows. At least with a 10m option a guild that has 17-18 people and isn’t able to fill, can still participate. But maybe you are afraid that more people can achieve CE and you feel devalued? Better have people simply not participate due to recruiting problems, very smart.