That’s not why. Landmark isn’t an intellectual property designation. Nor does being privately owned by a business confer any intellectual property protections.
Trademarks protect things like logos and phrases. A specific graphic representation of a building can be trademarked but it’s not the building itself. Trademarks are unique from copyright and patents in that they can continue to be held so long as they are actively used.
Architectural works are protected by copyright, which means the protections eventually expire. This is also a more recent development in intellectual property rights, particularly in the US which is why Disney’s Hollywood Studios and Universal Studios Florida are full of replicas of buildings from around Hollywood. It actually would be permissible to build a replica of Cinderella Castle because it predates the changes to US copyright law protecting the actual building itself. This is also why the story about the Chinese Theatre rights was not true.
The London Eye is recent enough that it is still protected by copyright. Victoria Tower and the Eiffel Tower are both old enough that any copyright protection that would have applied has expired. The Eiffel Tower is famously the source of copyright confusion as while the tower itself is not protected by copyright, the night show that was installed in 1989 is protected by copyright.
Disney could easily add a Ferris Wheel and people would know the association. Merlin doesn’t own the general concept of the ride or it being in an urban area. The issue is more that’s it’s just not a great idea.
