You also have an element of Global business strategy going into play here too:
1. Nick V will only invest if it guarantees 25% return on investment (as quoted in an interview he did last year). This was not the case for the last 2 big investments, so there is an element of caution with Thorpe. Once bitten, twice shy, third time reluctant.
2. The strategy of Merlin is to add bedrooms and hotels. This is where the majority of the investment is happening because this is where the majority of the capital comes from - the rest of the investments are kept low key where possible and the parks hardware investment has ground to pretty much a halt except for special circumstances. It is safe business / revenue generation. Thorpe is limited to what it can do in this space because of location, available space etc. Compare Thorpe Shark to the hotels at Windsor, Chessie, Alton and Gardaland. As a result, you have a park that seems neglected because they aren't getting huge investments for hardware (other than DBGT) nor are they getting the hotel investments either.
3. IP's generally work. When you get the right one. Peppa Pig in Europe has been a massive hit at the parks and the World Of Play new openings are huge around the world. Sadly, Merlin will never have the UK license so has to settle for other IPs instead. Saw broke the mould and was a success. But piggy-packing on some-ones IP is less risky than creating your own - arguably the lazy, "quick win" option, but if it works you're a fool to not exploit it.
4. Thorpe seem to be becoming the Six Flags option of Merlin. They are going down the generic, lifeless, soul-less way of running a park as an experiment because they have no specialism - Chessie with the zoo; LEGOLAND with, well, Lego; Alton and the Towers, Gardaland being a bit like Disney, Heide Park being beautiful etc, Thorpe has no anchor and while it is so close to Chessie and LEGOLAND I don't think it ever will have one. But, they are now stuck in a rutt of this way of working doesn't work andno-one is willing to give up the capital to break them out of the cycle because of the toxic "you must perform financially in order to be given money to better yourself" attitudes that modern businesses have. I'd love it if Merlin were to just say "Right, here's £100 million. Sweep the entire park and make it great again within a year so that we can ramp up profits next year" but it won't happen.
5. I have a feeling that the new "owners" will not be as restrictive as playing shareholder politics. They are going to be investing for the strength of their business and not the perceptions and opinions of shareholders. This may see an uplift in investments in due course. But, don't hold your breath
6. MMM seem to play it safe with investments and there are no rumblings anywhere of us working with pioneering or elite ride manufacturers. And while Merlin have the perception that rollercoasters bring in more people than flat rides (which are by large more expensive to maintain for the investment they require in comparison), we will always see the decline in flats and ramping up of coasters of varying quality. Maybe if they were more daring, we would see an RMC spring up at Thorpe.. or a Mack Launcher at Chessie.