Well this is demonstrably untrue. Merlin have built a total of 11(ish) coasters in the UK.
They've created at least 14 other attractions at Alton Towers alone during their ownership.
I slightly exaggerated yes although clearly said with some exceptions.
With Toxicator being the first thrilling permanent flat ride at Towers in years and Thorpe Park's last thrill flat ride is Rush which was in 2005.
Looking at this, there were a number of rides being built during Merlin's ownership with a focus on children's and family rides mainly at Legoland, Chessington and Cbeebies Land.
I was right with some exceptions when it came to thrill rides, more major non coaster rides and especially water rides.
With empty sites on park, glaring gaps in the line up and no rides that can get you splashed at Alton Towers specifically, it is concerning when a lot of the whole family and thrill focus was coasters.
I'm hoping Toxicator is a turnaround with this and we see a few more flat rides for both thrill and families in the future.
Rollercoaster wise under Merlin we've had which you've rightfully pointed out is 10 rollercoasters (11 if we count Minifigure as 2).
Thirteen
Smiler
Wickerman
Octonauts
Swarm
Saw
Hyperia
Mandrill Mayhem
Minifigure Speedway
Duplo Dino Coaster
Other rides we've had without counting secondhand rides in the portfolio:
Toxicator
Nemesis Sub Terra
Croc Drop
Barrel Bail Out
Mamba Strike
Ostrich Stampede
Kobra
Zufari
Flight of the Sky Lion
Fire and Ice Freefall
Mia/Autumn's Riding Adventure
Destiny's Bounty
Big Easy Bumpers
Train in a Shed (Ghost Train)
Numerous Cbeebies Land Rides
Possibly further rides I may have missed.
Counting secondhand rides:
Storm Surge
River Rafts
Trawler Trouble/Timber Tug Boat
High Striker
Retrosquad and Flavios is not counted as they were pop up additions.
You're right yes in that they haven't just done coasters but my point does reflect that we haven't seen many water rides, flats and dark rides with some exceptions.