MMA might be more popular in the United States and a few European countries but around the world boxing still has a significant presence. Tyson Fury fought in front of 94,000 in attendance at Wembley Stadium a couple years ago, Naoya Inoue sold out the Tokyo Dome earlier this month. We’ve had some big fights involving Canelo, Inoue, Fury, Usyk and Lomachenko in the span of a month, which had a lot of buzz on social media so it’s kind of crazy to think people aren’t watching boxing anymore.
Boxing is still popular and I hold it based on gate receipt and PPV no top MMA fighters can claim that they earned more than the top boxers, Ngannou earned more in his one fight
Francis Ngannou earned $20 million from his fight against Anthony Joshua.
For his fight with Tyson Fury, he earned $10 million, that's $30 million for two boxing matches.
He made $3.5 million from 14 UFC fights in 7 years.
Here is the breakdown of Ngannou's earnings both on MMA and boxing
https://www.givemesport.com/francis-ngannou-net-worth-boxing-ufc-earnings-purses
Back on Wilder, I don't think it is a money matter he has too much of it, every boxer wants a graceful exit if he wants this to be his last fight or if he still wants to get even on Fury or to share a ring with Joshua to fill up people's curiosity on who between them is the best, they've deprived the boxing fans so I'm sure Wilder and Joshua will still want to get it done.
That cannot be denied, Ngannou is very popular in UFC and promoters have seen that they can make a hype if he'll pair him with Fury in the ring and they succeeded it. And that result on his fight with Fury which was a controvertial one had help him increase his value, until he loss to Anthony Joshua.
I think the hype for him is already over but if he'll retire already, he can live a good life as long as he know how to invest his earning.
The biggest money are on the heaviest division, no wonder fighters like Wilder is still fighting despite already losing in most of his big fights after the loss to Fury.