Never mind, got my answer in the MGS forums. No way to start a new game, aside from deletion of your existing file. If you game on PS4, of course, you have freedoms to back your own save to USB, so crap like this can be avoided.

So, I am now at Mission 48, and if you aren't at that point, please do not continue reading, as I do not want to be responsible for spoilers!

BTW, I hear the game ends after 51(?), assuming no important side ops pop-up in-between that time. That being said, I have to ask -- is there a New Game mode? Or New Game Plus? Or do I have to delete my existing save to start over? Along with all my FOB progress?

Reason/s I ask? I am sure you are well aware..."QUIET!!!"

Yes, I am willing to restart to get Quiet back!!! She's gone, but I was told that if I wanted to 100% the game, I had to let her go, as her story prevents progress to full completion. Her story was an amazing one, albeit...but there is no reason why a buddy should hold your story progress for ransom -- "Either complete my story, or don't progress..." it is rather tasteless and upsetting. Of course, I could always not use Quiet, keep my save and ache the entire time, wishing I had figured out how to get that blasted ricochet skill to actually work, with such a neat concept that it was...all while hoping Hideo Kojima releases an update that patches her back in, or adds an extra story leg to the character. I mean, he knew that she was going to be so hotly received, from footage he showed us freaking two years ago...why would he proceed to axe her out? I'm assuming he's not that dim, or cold hearted. for the sake of this game, after one beats it, you should be able to revert back to a "pre-story" completion world state, where everyone still exists.

...and if that wasn't enough...SHE IS EXPENSIVE? All that maxing out, defeating her for the Sniper Wolf gear, all the millions I've put into her...what for? All so she can disappear? Why even create customization options? Let us know she's a temporary asset by not allowing for all your expenses to go into her. That's why I have to believe she is coming back...or can come back, it just makes no sense. However, if I learned anything from other games, such as, Witcher 3...I am not hanging on to some hopes of an NG+ or added content on any matter, as any fix may well be 3-6 months down the road.