I think Usyk would rather retire than take an easy fight against this mandatory challenger. Honestly, Fury is a much tougher opponent than Parker, and that’s who he’s facing next. Since Usyk has hinted at retirement before, I wouldn’t be surprised if he decides to hang up his gloves soon, there’s really no one left in the division who can truly challenge him.
Before his fight against Dubois was announced, Usyk mentioned he wanted to fight two more times before retiring (or maybe said something along the lines of having only 2 fights left in him - slightly different meaning but still). And I believe he confirmed after the Dubois fight that he's not retiring. If Parker is the best option for him, I think he will take it, but I'm not sure if it is, especially if it would be the last fight in Usyk's career.
Then again, Parker might not sound super-exciting but, other than being the mandatory contender, he has a good recent record and, unlike other potential candidates, he hasn't fought Usyk ever before.
But it is starting to look like the real choice is between Parker and Fury. I think Usyk doesn't care too much about vacating belt if it comes to that, and will probably go where the money is better.