Tonali may look like an enticing player to have now but the situation looked a lot different a year ago.
We had Brozovic and Barella who were definite starters. We had Sensi who was coming from a tough season but there was hope he could regain his fitness. There was Eriksen who had joined just 6 months earlier and there was optimism he would have a much better second season once he adjusted to Conte. And, most importantly, Tonali was never a Conte request. Tonali was always supposed to be a Marotta signing for the future. Tonali or no Tonali, Conte had asked for Ndombele, Vidal or a similar midfielder. So a year ago Tonali looked more like a luxury signing.
We also need to remember Tonali was actually quite poor for Milan last season. He got lots of playing time (mainly thanks to Bennacer injury), he worked on his game and now he looks much better this season. Would he have got similar playing time at Inter? Not really. He wouldn't have benched Brozovic, Barella, Vidal of first round or integrated Eriksen of second round. He would mostly get minutes from sub appearances and on the rare occasions one of the starters was missing but this isn't how you develop a youngster. The only period he could have got playing time was during the weird phase of the season when Conte was starting Gagliardini. But even then it would depend on Conte's trust on his abilities.
Things are a lot different this season and that's why he looks enticing for us. Uncertainty around Brozovic future, Eriksen is semi-retired, Sensi is broken, Hakan has been poor and Vidal has been inconsistent when he isn't injured. So it would be nice to have Tonali now. But the real question is: Would Tonali have been the player he is today if he had rotted on Inter's bench last season? I seriously doubt it.