I guess it comes down to expectations. What have Merlin done over the past 10 years to make anyone think that their attractions should be classed as world class? Even moreso over the past 5 years.
Merlin's investments do the job. They're better than quite a few parks, and at an individual park level their larger investments are more regular than quite a lot of other parks (although it has stretched out further admittedly). "It's fine" sums it up quite nicely.
We can want Merlin parks to do fantastic things. We can want every park in the world to do fantastic things. But honestly, Merlin don't hit the fantastic mark often, so it just feels unrealistic to expect that. You could argue it's a pessimistic outlook, but equally it's quite realistic imo