I think Fury's antics are about both money and mental health; in terms of 'fear' in the ring, I don't believe that for a second.
By mental health, I'm not talking about depressive states or anything of the sort, rather, him always being unstable, volatile and predictably unpredictable in his antics, behaviour and patterns to get practically any fight made. It's not exclusive to Usyk, and come and gone, he'll be the same further down the road until he retires. It's part and parcel of what he is.
The money part should be obvious. Since the Middle East got involved in boxing, nothing has been the same; if you can procure the bag there, you're making 3-4 times what you will anywhere else, morals be damned. Fighter splits and the rest of it are immaterial compared to what the pursuit is really about, which is getting the fight made and going ahead there. Take them out of the equation and everything falls back in line. If Fury can get a fight in the M.E. he'll take it over the noble, traditions of going elsewhere, unless someone else is willing to put the same kind of package forth, which they aren't.
Usyk is the fall guy in all of this, but not because of actual boxing as opposed to what Fury is and what he's in pursuit of; you can put <insert name> of any other opponent whom he believes he should be getting obscene money for and nothing would be different to what we're seeing.
Ultimately, like football, 'legacy' is meaning less and less to these athletes over acquiring eye-watering (even for them relative to what they previously earned) amounts of money.