A couple things:
1. I think the Sparrow Movement will be critical for the royals’ fate. That’s not to say the Sparrows win, but as we’ve seen, Cersei and Margaery’s fates are closely linked to trials by the Faith, Frankengregor’s plot intersects with the Faith, etc. So I think Tommen, Margaery, Cersei, the Small Council, and the Kingsguard will be tied up with the Movement in the final moments.
2. I think it’s more than joining Team Aegon. If Aegon is going to take King’s Landing easily – to better set up his sudden reversal at Dany’s hands – then the Movement is what opens up the gates for him and allows him to win because they can deliver public legitimacy (especially if the Lannisters are publicly discredited at the same time).
3. I then think everything comes crashing down on them in the person of the dragons. The Faith has never really had an answer to them short of the storming of the Dragonpit, but Dany’s dragons aren’t going to let themselves be chained again.