Somebody is clearly doing money laundering with these small B&M coasters at Merlin. Like, why would such a park invest in a big, expensive piece of hardware for what will essentially be a kiddie Wing coaster with a (hopefully not) 1.4 m height restriction?
Surely they could choose other less expensive options as a stepping stone before buying a Rolls Roice.
B&Ms are reliable and have high capacities, each individual coaster is still fairly small (though will be the largest at each Legoland park). Kiddie/family coasters generally dont have particularly high capacity, as families/kids are not necessarily the main demographic for most parks - but Legoland is the exception, with a lot of visitors and the vast majority of them are young families, so getting a not-so-massive ride with high capacity is perfect.
In terms of the cost, Legoland parks might physically not be huge, but their attendances are high - Legoland Deutschland had 1.7million visitors in 2019. They must easily earn enough money to warrant investing in large scale rides.
I wouldn’t be surprised, though, if this is the start of adding larger scale rides to legoland parks, as Lego is such a popular brand, even outside of kids, they could start building thrill rides and/or larger dark rides and quite easily get other demographics coming to the parks.