it increases the amount of cards that can overlap between decks, which in some part undermines color identity, which is meant to promote diversity of card inclusion in the format. it's one of the foundational parts of edh. to me there would need to be an extremely important reason to diminish it.

I don't really care about design intent. I bet the design intent behind EDH has changed since only the goofy elder dragons were generals (& tuck, color generation…)
It's barely a format anyway, more like slightly structured kitchen magic. I care about the end result and all I can see is fun stuff*

so if someone is pro hybrid mana, but think phyrexian mana is too far, they need to make an argument for hybrid mana that does not justify phyrexian mana! and developer intent is not that argument

But... that's just opinion.

Phyrexian or Hybrid mana intents being good or bad are just opinions.

I COULD be swayed to be more lenient UNDER 1 CONDITION: we revert the rule regarding producing colors outside your Commander's CID.

If that rule is reverted, I'd have no problem with hybrids, or even splits/Adventures where only half of them is correct, because it's literally impossible to cast.

I'm of the opinion of keeping the current rules, because it causes the fewest issues. I would probably have no problem with someone having Rhys the Redeemed in their monogreen Elf deck's 99. But that's a Rule 0 discussion to me. I shouldn't EXPECT him to show up.

However...

Of course, the other problem with the Deans is that if you're casting the back side and only have the intent of using the back side... but your opponent blinks it... it comes back front-side.

To me, Phyrexian is the same as Twobrid: It's either colored or colorless.

If we're allowing Beseech the Queen in any deck, we should allow Phyrexian mana in any deck.

And... there's discussion both ways on Twobrid.