As for this incident: very sad
I've been going to Drayton Manor for about 26 years. That ride must be, I guess, about 20+ years old. Even assuming a low estimate of 1,000 rides per day, 120 days per year, that's about 2.5 million rides before this death - and probably much more in reality. It's a safe ride that unfortunately carries an element of risk when used incorrectly (standing)
I can't imagine them adding seatbelts - might introduce further risk with people being unable to operate them. They might increase age limits for unsupervised riding. Or simply increase the size of warning signs, and have staff reiterate "no standing!".