No one doubts Modric's quality; he's one of the best midfielders in the world. However, his advancing age will significantly impact his performance and physical quality, so I believe his contribution is somewhat less than optimal.
There's no definitive proof for AC Milan, but I've seen the arrivals and departures of several players in their squad not make them stronger, so I'm a little pessimistic about AC Milan.
Modric is certainly not the same as he was when he was younger, and he will certainly not play like he used to.
AC Milan certainly has a strong reason to bring in Modric. They want him to be a mentor for their young players. Modric has certainly achieved many achievements at his former club. Modric's extensive experience will certainly give AC Milan the potential to emulate Modric's excellent performances and numerous awards with his former club.
However, your opinion is certainly valid, but of course, now we'll just have to wait and see what happens. AC Milan should be stronger this coming season. We'll see what happens. However, I believe that with Modric, AC Milan's mentality will have the potential to be strong next season.