I get what you're saying in terms of how the MCU and LucasFilm have always been PG-13 brands at best (LucasFilm really is more PG, tbh. Star Wars just gets the PG-13 rating bc of the pew-pew "violence"). Audiences know what they're getting when the go to one of those movies, a Pixar, WDAS, or Disney proper movie. But FOX has an identity too. Their known for PG-13/R-rated fare and just because the X-Men are going into the MCU, I don't see why Deadpool couldn't stand alone outside of that and be R under the Fox brand? Audiences EXPECT Deadpool to be a pretty hard R, not MCU's PG-13 formula.
At the very least, if they're gonna take Deadpool and put him in the MCU, they could let Ryan Reynolds finish out his trilogy. You can't argue with box office and the worldwide totals at $783M WW and $785M WW for 1 & 2 and you're gonna be pissing off a lot of those moviegoers who liked him as an R-rated character. I understand Disney's thinking is that "well if Deadpool made that much with an R rating, imagine how much the character would make with a PG-13!". And they may be right, but it would be at the expense of what made Deadpool 1 & 2 so enjoyable.