It's not the 40's though, that's not going to happen ever again. Obviously, 20 fights bring around a hell of a lot of personal sponsorship too, which probably doubles that.
20 fights at a 10m purse (which is low) @ 50/50 is £100m, double that for sponsorship and that's ~£200m, is that not enough?
Those 20 challengers would need to be worthy of a title fight, and likely have to go through a boatload of lower-paid fights to get there. Then keep in mind the guy they're fighting has beat 20 other top fighters, so it's unlikely they're going to win, and unlikely that anyone else has had a time at the top (and got money for that).
The lower-level fighters are risking their lives too, they're probably risking more as they're less skilled and continually fighting guys who are ranked higher than them.
Champions might not like it as much, but the other 99.99% would, and the champions would still end up with far more cash than anyone else.