I am not sure why people are so sure about Morris being a true PF at 6'9", while they couldn't imagine Tatum (also 6'9") or Hayward (6'8") playing the position. Morris even came to the team as a SF and his game greatly resembled that of a SF.
Your main point here is a good one, I think. His game is a wing's game; he's not a "true PF". His offensive rebounding numbers, for example, have always been minuscule (and his defensive rebounding has never cracked 20%...).
Having said that, the main difference is weight: Morris 235, Hayward 225, and Tatum 210.
I agree that having Horford's defense helped mitigate any issues in the front court, but at least Kanter (or Poirier?) are very big individuals who should be able to bang with the best of them down on the low post. Everybody else should be switching like crazy on the perimeter.
A bigger priority, I think, is pick and roll defense, and here, at least, I think that Daniel Theis has an advantage (perhaps I'd feel differently if I knew Poirier's game better).
I personally just don't see any way we don't have a starting line-up that includes both Hayward/Tatum at the Forward spots going into the season. It may not last, or a trade may occur, but these two are by far the best options on opening night. Starting anybody else could once again cause discourse similar to what we had last season.
Meaning, I take it, that one of those two is the swing? Perhaps you're right.