Sorry, not convinced.
It doesn't make sense to let go of a highly rated young CB, when one of your 2 starters is 30y.o injury prone, and the other error prone. He'll see enough game time that will facilitate his development, without the pressure of being a starter. By keeping him, not only do PSG retain quality depth, but also a future replacement for T.S, without needing to dip in to the market in a few years.
And if you say that Marcos atm is not good enough for PSG, how is he good enough for Barca then? A club vying for all the available titles have to take in to account both short and long term. So having a 20.yr as an understudy makes more sense. Just like I'd prefer to see him at Barca, as an immediate back-up to a more estd CB, and have him gradually take over.