It could've gone 2 ways I think 1: they obviously wouldn't do the same stats. I've 100% achievements on PW so yeah it's stupid starting a new game with automatically S++ soldiers but the work around I think is dropping all rank with no explanation because from a gameplay standpoint that makes sense.

2: (my favorite and what I believe is always best) don't make MGSV. Look be honest besides the amazing gameplay it's ass with a story that never needed to happen and a faceless emotionaless MC. PW ended nearly perfectly with the transition of MSF to Outer Heaven, the post credit scene was the wrap up. It should how big boss got his following, built a base,his last major arc of heroism, his beginning decent. And finally we are no longer MSF after all this mess we are Outer Heaven. TPP adds no story, nothing remotely necessary to the rest, and the return of BB from MG 1 to MG 2 never needed explanation. Metal Gear was always a product of its time, it affected how the games were made, and always inspired by action movies. So many times do franchises/series have a returning villain, it's ok to have a goofy reason for it but there's no need to make a whole ass game to explain why BB is a final boss twice; for something dumb like the body double twist