How big a failure would it be for Milan to not even finish top 4 again?
Depends how you classify failure, I suppose. If you're at all familiar with how disasterous their ownership has been in recent memory, you'd probably be a little more leniant in what is essentially their 1st year under stable and non-fradulent owners.
UEFA is punishing/restricting the current ownership because of the mistakes/incompetence of past regimes. It's very hard for them to make a giant leap when FFP is preventing them from splashing significant cash.
Now, it could be argued that they've spent enough to be in the top 4 despite these restrictions, but football isn't that simple, and Atalanta deserves to be where they are. Juve, Napoli and Inter are all comfortably better sides, and were thought to be before the season started.
When you say words like "again", it's grouping all of Milan's recent poor seasons together, when there should be clear lines in the sand between Berlusconi , Yonghong Li, and now Elliot Management's regimes. Make no mistake, Li left the club in shambles and much of the past year has just been cleaning up the mess as much as possible.
There's rumours saying they'll sack Leonardo if they don't make CL this season, and imo, that would be a mistake. He's done well cleaning up the roster and bringing in young talented players despite the FFP landmines placed in his path. For me, moving on from Gattuso would be the only massive change I'd make and I'm not entirely sure I'd even do that unless a
clear, significant upgrade is coming in. They have a good young core that just needs to grow together and be supplemented as time goes on.
Their return to the top was always going to be a slow, gradual rise. Anyone who thought otherwise wasn't paying attention or is incapable of processing circumstance. That aside, they're in a much better position today than they were this time last year and there's reason for optimism.
If you want to look at a true failed season, look at my club.