Yes, you are correct. But like what
@Moxie has said before this post, he is only 20 years old turning 21 this upcoming August. So, he has a lot of time to improve his game and physical tools this year and off-season. In addition, he switched coaches right after AO this year. Furthermore, he got an illness and some unlucky injuries right after reaching AO QF.
Sinner might not become an all-time great player trajectory like Alcaraz, but the most important thing is that he improves his game steadily.
I would rather have Sinner get stagnant now while he is 20 years old than when he is 25-27 years old.
Like I said before, Sinner is similar to Andy Murray when Murray was 20 years old. Murray was skinny and was trying to becoming more consistent on the tour. Then, he bulked up, and he started to win more consistently.
Now, Sinner does not have the defensive ability of Murray or even the tennis skills, but
Sinner has the talent and especially the mind to become a very good player in the future assuming he continues to work on his game and physically tools consistently.