The longest lasting AW campaign I've run capped out at 35 sessions. Of the five original characters, three were still in play (with the other two players having lost a character to death and retired one to safety respectively). We could have gone longer, but the campaign was at a satisfying end point so we wrapped it up. But here's the catch - the players explicitly wanted a slower progression to give them a chance to "inhabit" their characters for longer than you typically et in AW (where 10 sessions in you might be looking at "retire to safety" and "switch playbooks" as those among your dwindling supply of possible advances to left take). The only change we made was to double the amount of XP needed for an advance, and it worked perfectly. As a result it told a "slower" story, but that was by design.
Now, in terms of how to address you specific issue, I think one of the best things that can be done is to "put your bloody fingerprints" all over the allies. You're no longer looking at them through cross-hairs and they are no longer actively looking to mess with the PC - but that doesn't turn them into automatons. They're not suborning their parts to the Skinner's every whim. They still have wants and desires and needs and problems, and just because they're your ally doesn't mean those all stop. Indeed, being the PC's ally might make them worse. What do you do when your allies are competing amongst each other for your attention/favor/love? That warlord who you've turned into your love-toy - how do his goons feel about him "going soft?" Does he still command their respect? So sure, you can turn the Humongous into an ally, but doing so might intrinsically change him, for better or for worse.
As the MC you don't want to be a jerk and undermine what your PC has worked to achieve, but that doesn't mean you need to make their lives sunny and uncomplicated either.