Think you're getting a very skew impression of enthusiasts online from a relatively small representation, or online exaggerations. Enthusiasts on the whole don't hate theme parks, unless they're very confused and getting themselves wound up. It's because these parks could be fantastic today and so much more than the way Merlin develop/run them.
In the wider picture, I think it's clear that a park made up of a patchwork of IPs and underwhelming attractions will make for a very commercial feeling, mediocre day out. Merlin do it because they don't care about parks long term or even mid term. They just want whatever cheapest option can be used to boost sales that year.
If guests respond better to cheap attractions cloaked by a well-known IP slapped on top, then that only makes a case for the decline of the theme park industry. Been shown over and over again that there's much greater value in originality for parks and guests in the long term.
The Merlin parks are fast becoming quite esoteric in terms of actual entertainment, relying on growing a 'Merlin brand' following with the beguiled public, rather than simply entertaining guests and giving them a good experience.
Europa's IPs you mention are either very self-contained areas (Arthur) that actually live up to the IP, or are very minor attractions. They don't make up sizeable parts of the park or replace classic attractions with IPs at every opportunity.
Also I think in the bigger picture, enthusiasts were glad to see IL Bubbleworks go and had actually been wanting a replacement for years. It was appauling that IL Bubbleworks ever happened to Prof Burp's in the first place. And that thank god Gruffalo was a good ride in the end, if dull for what a family dark ride could be. Room On The Broom will probably be similar.