I'm thinking the same as you, he will do well in these ATP Masters tournaments but to win a grand slam, I can't see it. To break into the top 10 is very hard as you say with Nadal, Federer and Djokovic still playing and with the young talent already in the top 10 it will be very difficult to see him break in unless one or two of the young talent go down with injuries.
It reminds me a little bit of the Stan Wawrinka situation. Can I remind you how old Stan Wawrinka was when he won his first Grand Slam title? Australian Open in 2014 at the age of 29. Then there was Roland Garros in 2015 and the US Open in 2016. Therefore, I would not be so categorical about the TOP 10 of the ATP tour, nor about winning a Grand Slam title. If Karatsev will not be haunted by injuries and he can confirm his level this year and next year, then age will not interfere with achieving the highest goals.