Deku and Shigaraki are both on similar paths. Deku to become the greatest hero and Shigaraki the most powerful villain. Sure, the story built up AFO in the beginning because that's all All Might knew and AFO was the biggest threat. But, both Shigaraki and Deku will surpass their predecessors one way or another and as powerful AFO is, the gap between him and Shigaraki is closing, just like the gap between Deku and All Might.
Horikoshi did a smart thing showing us All Might VS AFO. We've already seen AFO get beat, and he was beaten by a One For All user (twice if you count their off-panel fight years ago). That chapter is closed but it doesn't mean AFO won't try to get back on top again. That's where the quirk singularity and ultimate being stuff comes in. I think AFO is building up to make a big play that will force Shiggy to kill him. Remember, both Deku and Shigaraki are on similar paths. I don't see Toshinori dying without AFO doing the same and the only one with the power and (eventual) desire to do so will be Shigaraki.