Apparently the fight got 800,000-850,000 ppv's up from 325,000 for the first encounter.
Imagine your humiliation being brought to a whole new audience who've not had any interest in you before, and post-fight, you've put up the kind of excuse that makes you a worldwide meme.
Wilder taking on a third fight, and potentially getting absolutely destroyed again would be the kind of embarrassment and fall from grace a man like him cannot come back from as any kind of 'celebrity' the American sports shows have been absolutely tearing the guy to pieces for getting beaten up in such a fashion - and that was before the weighted outfit excuse.
The guy has some serious thinking to do on his next move in terms of activating the clause. The money he'll make is, on top of what he's already got, enough to set him and generations of his lineage up for life, but it will come with a consequence, and he, as a proud man and someone who prides himself on being a fighter (not a boxer), will have to measure that up against the extra money. Becoming a living meme and joke for the next few years, if not all-time, is one hell of a fall from grace from where he's been, via whatever means (padded stats, top fighter avoidance), and something to factor into his decision (from his own POV, not that of a layman who goes: 'wow! $20m!') that, too, with the potential of having the difference between he and Fury indelibly stamped in his consciousness, should he take another absolute whooping.
Trying to think from his perspective, it seems to be a tough call to make that puts everything on the line for him. Would give him kudos for opting in if he is considering all of the above.