Parker isn’t a hard enough puncher and doesn’t have a good enough gas tank to keep relentless pressure fighters off him. Parker is good enough to beat most fighters with low output/poor stamina because he has solid skills and defence. That defence tends to go to pieces if he is put under too much pressure. Basically explains why he beat Zhang and Wilder but lost to Joyce and struggled with Chisora and Whyte. Chisora and Whyte both have more ‘variable’ output within a fight than Joyce, which explains why Parker had good and bad spots against both.
Styles make fights its that simple. Plus Joyce is a shell of the man he was before zhang took his soul. Joyce was on a march to a title shot and at that time he had every chance of beating a fury or AJ. Poor matchmaking with zhang ended his career. Remember the first zhang fight how much punishment Joyce took off an absoloute monster puncher before it got stopped. That's how much it took to crack the egg but once it's cracked it never going to be the same again. Most of the top 10/15 maybe 20 is capable of beating each other there's no real shock in any of those potential matchups with the only real exception being usyk.
Boxing is all about timing, if they fought today Parker wins... Fury for example would lose to 3 or 4 of the Top 10 now aday.. where as a few years ago he would be them all.