I think most people would probably prefer using their own online services Netflix etc. vs an onboard offering, especially if the Wi-Fi is free.
The Wifi isn't going to get good enough to manage a hundred or so people streaming content from their own services. The on-board systems work by having the content stored on-board the train on a media server. Because the data is stored locally, the 4g or 5g connection from the train to the outside world doesn't get used at all, and the only thing that has to work is the WiFi connection in the carriages, which generally isn't the problem anyway.
The current LNER trial it sounds interesting in that it replaces whatever bunch of dross has been selected and bought by the train operator (or the service they are rebadging) with an edge server for ITV Hub, and potentially more interesting operators down the line.
In principle, edge servers can be updated via a mix of push and pull. Push means that whenever ITV Hub adds popular new content it gets sent to all the trains, ready for people to watch. Pull means that if you choose to watch something a bit more obscure it will download it on the fly (keeping a copy in case anybody else wants to watch it later). Because the majority of people are watching the same 100 or so programmes at any one time, the amount which has to be fetched overall is much lower and can be accommodated by the 5g connection.
The really great thing about providing edge servers for popular networks is that you are using your own login and your own subscription, so they aren't paying for any rights at all. The logging in bit, and the bit where your browser or app gets a key to unlock the video, does have to go over the 5g, but that's a really small amount of data.
This is a really interesting way forward for the industry. I'd describe it as a new way forward, but the first time I heard about it was about 15 years ago when the BBC were proposing to put a copy of all the iPlayer content in every telephone exchange to save bandwidth. It's been spreading out across the internet for years, but it's now become relatively easy and safe for the various services to just provide a the software to do the job in this kind of environment.