Well first off, most people here don't play competitive.
Irrelevant, otherwise we wouldn't need balance patches and commander reworks.
You'd be going exclusively against meta or using your own non meta, being clapped hard and spammed forums asking for balance and nerfs, leading to new meta and so on.
Second, if indeed there are only a few meta combos then what's the difference going to be here exactly to the competitive scene?
For one, due to how decks work in AoE3, only about 30% of cards are non meta.
With how it works in CoH2, 95% of all abilities would become irrelevant and at this very moment, in case you have yet to notice, we're working towards making as many abilities viable and desirable as possible, which is not the case of custom builds stacked all with meta options exclusively.
Right now there are a few meta combo commanders and that's about it, as you said if you go something else you lose.
In case you've missed it, we're in the middle of the process of changing that.
And even within the current meta commanders, not all 5 abilities are top of the line when it comes to potency, contrary to stacking custom one with best stuff only.
So in my eyes it'd be a win-win situation, the competitive people still have their "award winning" strats to play with while more casual players, which are again more numerous, get to experiment, roleplay, have fun and so on and so forth with the rest of the customizable options.
For you, sure, you're just a comp stomper, literally anything goes for you and you can't blame balance for losing to AI.
However for people who do play pvp, facing the exact same thing all the damn time is the worst possible outcome ever, again, this is also we had balance patches for last 8 years.
There is absolutely no reason why we couldn't have this system in play by now other than the time for it being way past at this point unfortunately.
Sure, if you deliberately choose to ignore all the reasons against it, I guess there is no reason not.