The biggest issue for Northern parks like Flamingoland is quite simply their location.
FL can be a three hour drive from Stafford. Now, it's fair that Alton is also a three hour (plus) drive from North Yorkshire and farther North. However, Alton is also three hours from London. Four hours from Norfolk, four hours from Cardiff, etc.
Even if Alton Towers was in Malton, gate figures would simply not be as high as Alton's because it makes the trip to the park a full day. While it can be a long way and a real trek to get to Alton, it's always just inside reasonable for one day.
Merlin aren't worried about their unholy trinity down south. They have a massive captive audience in London (which Alton also feeds off).
I think Drayton is very lucky to be alive so close to Alton. I think they invested just right, at just the right times to keep them afloat (which American Adventure failed to do, or simply couldn't).
I don't think the Unholy Trinity are competition for either Alton or Drayton, and certainly not FL or LWV. I don't even think Drayton is really competition for Alton, they're just the cheaper version, with enough to make them worth visiting (let's not get into which is actually the cheaper day otu based on bogoffs, food, etc
).
However, the presence of the other parks are all vital to the active competition. Flamingoland is unlikely to be stealing customers off Alton, but if it's getting all the roller coaster media attention by unveiling Europe's tallest, steepest, fastest and better than Boulder Dashist wooden coaster (hint, hint Nick) then Alton are caught on the back foot. They don't want people discussing other parks because it makes them forget about Alton. Likewise Thorpe...
So the competition keeps the parks from getting lazy. Flamingoland I suspect are in competition primarily with LWV and it could be argued that's pushed them to excel the way they have. It may not be much competition, and it's a bit like kicking sand in a baby's face, but it will encourage them. Likewise, if they can expand enough, they CAN compete with Alton by offering the dreaded "resort" option to visitors.
So no, the country wouldn't benefit from less parks. It would be rubbish. I don't think we need any more, but some of them certainly need to either get investment help, or better management in to bolster their national appeal. Come on Oakwood, get with it