I actually think that as much as the 1.4m coaster probably seems like a strange choice on face value, it actually makes a fair bit of sense to me.
Think about it; it rounds out the park quite nicely, and I’d imagine Paultons Park is probably scaring Merlin a lot more than some people expect in terms of competition to Chessington, so one logical way for Merlin to beat Paultons coaster-wise is to tread on turf that Paultons hasn’t yet tackled, and the only coaster turf I can think of that Paultons hasn’t really tackled yet is a 1.4m thrill coaster.
In terms of Thorpe, Chessington building one thrill coaster doesn’t necessarily render Thorpe redundant. Despite the 1.4m height restriction, I get the impression that this will be quite a tame ride in thrill coaster terms, similar to what Air was intended to be at Alton Towers. Remember, Chessington’s target audience goes up to children aged 12; the older end of that age bracket will surely be hitting 1.4m and gaining an appetite for slightly more thrilling rides. I hit 1.4m at age 10, and I’ve never exactly been tall at any stage of my life (at 18, I’m now around 5’9”-5’10”).
Thorpe doesn’t really have anything in the way of a “starter thrill ride” so to speak, so this ride doesn’t infringe on Thorpe’s turf per se. As much as it’s strongly rumoured to be the same ride type as Swarm, I think it’s going to be a milder ride than Swarm and have some quite different elements, from what Chessington have said.
Besides, I’ve heard rumblings that now they’re under new ownership, Merlin plans to bring investment to all of the RTPs in order to aid their recovery from COVID. They said something to this end at the Project Amazon consultation, and the mood in the room was apparently that they weren’t just referring to Chessington, they were referring to all the parks. A coaster project is apparently in the works for Thorpe, and will probably open at some point within the next few years.