While I do agree that Ash's jump to World Champion level can be a bit sudden, I see that more of a flaw with how the previous series showed the gaps as well, like that Flint episode
Ash needed to level up to their scale someday, and what better way to start then after he finally becomes champion of Alola
Skill wise, his growth has been pretty realistic. Top 16 - Orange island winner - Top 8 - Battle Frontier winner - Top 4 - Hitting a slump and Top 8 - Top 2 - Champion - World Champion
The ISSUE is with the fact it's a new team. Even that could have been avoided if we saw more organic interactions of the new team earlier in the series and more training segments and a mention how Ash is training harder than ever
So to summarise, Ash's skill as a trainer being world champion level makes sense, his new mons being as strong less so. But also, even if the execution was wonky, the decision to have Ash finally ascend to this status in this series was absolutely the right one. It was ABOUT time and remember,
It would ALWAYS felt uncanny to see Ash become this strong and defeat everyone simply because the rest of the series did a poor job by constantly showing him as handicapped and not as strong, with some ridiculous gaps such as again, the Flint episode
So the execution may be botched, but I'm glad we finally are at this stage. It's about time Ash's journey let him more upwards and he won