Not necessarily. They can offscreen it and use it as excuse to show Ash less.
while that part is true DuskAstral nailed the other thing.
Battle Frontier is different, as Satoshi declined the offer because he wanted to continue traveling.
Being a Monarch doesn't prevent him from traveling, so why would he want not to take it?
Ash can still travel being a monarch, the battle frontier would've forced him to stay in one location.
The other thing is that the BF was treated more like a secret fight club, where only those who Scott saw worthy could challenge it, and is also for the most part in one region that being Kanto with Brandon being the exception of being in sinnoh right now.
The PWC meanwhile is a world wide tournament where everyone could enter, and it's gained Ash alot more attention than anything else he's done in his career, even winning the alola league.
combined with beating 2 champions, possibly 3 if he does beat Leon, I can't see him simply walking off, not only would it be disrespectful to those he beat, but I get the feeling a few of them probably wouldn't let him vacate the title if he tried like Bea.