But you told us on a post before this one that the MCU "have repeatedly hammered home the point that 'If it ain't broke, don't fix it.'" So with your suggestion that they change what they've seemingly perfected in the past (box office, audience and critical acclaim prove this), you seem to be saying the MCU is broken and that you clearly think it needs fixing. It also sounds like from your scenario that the MCU is the Snyder-Verse.
I don't think a lot of people would be agreeing with you about any of this.
That "ain't broke, don't fix it" lines were in reference to Deadpool. But inserting Deadpool into the MCU WOULD be unavoidably changing the formula by crossing over with an R-Rated property, so the saying applies both ways. They have three choices here:
1) Change the MCU by making it a place where R-Rated characters and films exist, inevitably raising questions/demands about why looser standards can't apply to other characters besides Deadpool.
2) Change Deadpool by rebooting and going back on their word to censor him, allowing clean crossover as yet another PG-13 franchise.
3) Simply move forward with Deadpool 3 as it was originally planned with adjustments to the plot based on plans they might have for some characters elsewhere. It is not part of the MCU nor be marketed as such, simply another movie that Marvel also happens to be producing, just like how Disney "technically" makes movies like "Pulp Fiction."
If you ask me, the choice here is clear. Keeping Deadpool separate allows the smoothest transition and holds the least risk for all parties involved, while maintaining the status quo that Feige so rigorously maintains.
re: criticism of the MCU - I have a lot of issues with it that I've unpacked fairly frequently, some of which they seem to be improving on, others not so much. That's ultimately immaterial to the question of whether Deadpool joins the MCU. As someone who wishes they WOULD make a space for R-Rated content, I wish he could simply join as-is (Option 1 above) but I do not think that is the path Disney and Marvel will agree on.
And yes, the Snyderverse was an attempt to create a DC-equivalent to the MCU. It fell to pieces because of studio interference and Snyder's deeply flawed vision, and they have since seen success in just greenlighting what works without being bound by an all-powerful continuity. It's blatant proof that NO everything does not have to be connected to be successful. Mandating that every output be intricately tied to a single storyline is only limiting.