1. He can win a slam, although I would say unlikely to happen next year.
2. No way, if anything the fact that he came so close and is rapidly improving on a surface which isn't his strongest should make him even more confident of winning RG on his favourite surface. He lost in round Robin last few years and it didn't affect his chances at all so why would it after losing a close final, plus its another 6 months away anyway, so much tennis to be played before this happens, so many chapters to be written before we get close to this part of the story.
Yes, Thiem winning Beijing beating Murray/Khachanov/Tsitsipas, couple calls shy from a Shanghai semi-final, winning Vienna, making the ATP Finals final beating Federer and Djokovic along the way but losing the final in a final set tiebreak, somehow, some way, negatively affects his chances for the next French Open...
Of course Tsitsipas will win a GS at some point, maybe not yet next year, but within 3 years maximum.
And as for Thiem, he is very disappointed right now, but after a good holiday, he will look back at a great end of the season with titles in Beijing, Vienna and a final of the WTF, so a lot of improvements on surfaces he did not like much, so won't affect him for the clay court season and RG.