On the matter of Oval, Trainline informed it's investors yesterday that it anticipates a loss of £150m of it's annualised net ticket sales as a direct result of that project, and envisages that it will be delayed until the 2026 financial year (which obviously buys them a bit more time, and commission!)
In any event, if Trainline removed their booking fees, it would be, by some margin, the best booking engine/planner in the industry and virtually all of the complaints would subside. Their UI/UX is very good, accessible, relatively simple to navigate and extremely popular. It's not perfect, but it is the closest thing we have in the UK to a 'national" app - since the decline of National Rail.
I'd wager that the number of rail passengers without the Trainline app installed on their phone is a very small minority. Even if you don't pay Trainline anything and book elsewhere, you've probably got the app.
Take into account all the TOC white label Trainline apps, and I suspect virtually all of the UK population that has ever travelled or considered travelling by train, has an app produced by Trainline on their device.