40% of the re-match purse still will many millions of USD$ but will it be enough to entice Wilder to trigger the re-match? He has well over 3 weeks to make up his mind, I am sure Wilder and his team will go over everything. Even if he triggers the clause for the re-match he might not be able to fight in the stipluated time-frame because it will take time after the operation on his ear to heal.
On the other side I cannot see Fury ever paying Wilder to not fight him even if it is part of the contract but it puts Wilder in an awkward position. As you said, if he does not fight Fury immediately via the contract clause then he might not get another opportunity to either fight Fury or fight for any of the heavyweight belts again.
I will be keeping a look out with interest for any news about this as soon as it breaks.
If he doesn't invoke it then he'll likely never get the opportunity to fight Fury ever again. If he didn't have a rematch clause Fury would have no reason to fight him and would move on to bigger and better things. The onyl reason Fury will fight him is because he's contractually obligated to if Wilder requests it.
Apparently Fury can possibly pay Wilder off to avoid the fight:
https://www.thesun.co.uk/sport/boxing/11043664/tyson-fury-pay-off-wilder-joshua/I think that would be silly though. Losing money compared to making boatloads on the rematch then fighting AJ after when they're both available. I'm not sure Wilder would take it either. He's set to make 40% of the purse from a rematch and then of course there's the huge perk of potentially winning the belt back.