I would say that Ademola Lookman has literally been unlucky with the transfer window this summer because first he was linked to Inter Milan, then Spurs, Bayern Munich, Arsenal and no one was successful. Staying at Atalanta is the only option at this moment and I doubt if he will be happy playing for the club for the rest of the period.
It is quite possible that by winter one of these clubs will decide to strengthen (or simply increase the depth of the squad/variability) and will still buy him, so it is probably too early to talk about an unsuccessful season. If this does not happen, then of course I would be very unhappy in his place, Atalanta will definitely not be in contention for anything this season, they have seriously weakened due to their actions in the transfer market.
Surely people will keep trying to knock on the door of Atalanta to try to get him, there is no doubt about that. But Lookman is losing precious time at his peak by staying longer and longer. If he were to be sold, he could have played at some better team and the results would have been better for him.
Not just financially, which I am sure he would make a lot of money over the course of years, but more about his age. He isn't some 22 year old kid anymore, he is 27 years old and losing time matters in this case. He will be probably 28 years old when he goes to a big team, at his peak, but then rest will slowly get lower. Sure footballers play a lot longer now, back in the day we would see people retire at 32-33 years old, and when someone played until 35 we would say that was a huge deal. So he still has 5+ years to play at top level, maybe even more. But the quicker the better, losing six months is one thing, but losing 1 year would be terrible for him.