I have a feeling that this movie would have received better reception from the fans if the whole anniversary event wasn't a thing and the main character is not Ash from another universe. I think it would have been better if this movie had a different protagonist with no similar characters from the main series. It would honestly change nothing in terms of the story and people would stop complaining about not seeing their favorite veterans and be excited for a story that takes place in a different world and at least see what a reboot would look it.
For that is the biggest mistake in general of all the anime: to continue used Ash as a protagonist, forcing a "continuity" (that does not exist), events and adaptations so that it can continue telling its history, establishing a Status Quo and not letting it grow Nor advance in anything.
If Ash had stopped being a protagonist 10-15 years ago and presented us with this movie I dare say that it would be much better received in general.