MJP is no more of a PF than Oubre. He averages just a hair more rebounds per game and that's being played as a stretch 4. You didn't explain why anyone needs this mythical PF that you've been griping about for the last year and a half. The only team left standing with a somewhat traditional PF are the Lakers and they play their starting PF as their backup and endgame Center. The Suns would be fine with Mikal, Oubre, and Cam as their main forward rotation with Saric for extra depth and to play a little backup Center. It's more important they build off of what works, which is the Oubre/Bridges duo at forward and Cam and Mikal in the bubble, because that proved to be successful for Monty's system, which you seem to disregard. If they don't add some throwback PF in the draft or free agency they still have a good squad at the ready to challenge for the postseason.
The most important pieces are getting Ayton insurance and a competent backup for Rubio. If Ayton wasn't suspended this year, the Suns would have made the postseason. They were a tiebreaker from it as it was, which is more than fair to say the Suns would have won at least 1 more game so tiebreakers didn't matter if Ayton wasn't suspended. So they need to bring in a backup C that can step up in case of injury or another fluke instance. Of course backup PG speaks for itself, things stall offensively when Rubio or Booker aren't out there to get things going.
Just FYI, Porter is 2 inches taller and 10-15 pounds heavier than Oubre.
And this odd bubble year is not something that should be used as a gauge of long term success in the NBA.
I am not saying they need a "throwback" PF...just someone who weighs more than 210 pounds and is capable of playing the 4, so that the team doesn't get bullied by opponents, as it often did this year.
The only other option is to go lightning fast, outrun opponents and kill them with speed (a la Boston). The problem there is that Oubre, Ayton, Carter and Payne are the only players on the roster with even above average speed/quickness, while several players (Cam, Bridges, Booker, Rubio, Saric, Kaminsky, Jerome, Baynes) are below average in that area.