In the UK you can get on a train without booking a ticket for that specific train, for example an open return or just a day pass. The train company has no idea how many seats will be taken or how many people will get on the train. So say it’s a 10 carriage train. Every seat is taken by someone, reserved or unreserved, and theres not a bit of standing room anywhere (this is very common). Which person sat on a reserved red light indicator seat should you kick out? And how do you know they didn’t reserve that seat specifically before you do that? Or do you kick someone out of a green lit non-reserved seat, with thier proof that the seat is not reserved and they are allowed to sit there, and your proof that you dont even have that seat reserved. It will also be the old people and small children sat down, and you won’t really be popular if you make them stand. Yeah you’re not sitting if you have this ticket. You’ll likely be stood by the entrance door for 3 hours instead. This seat reservation ticket may as well say, “sorry no seat today”, and it’s definitely infuriating to lean that you will be standing for your journey when you’ve payed full price for a seat, maybe £40 depending where.

Wouldn’t you be benefiting from your friends? It’s ok for a little bit, but if they live there permanently then they will pay off your mortgage and have nothing to show for it themselves. That sort of thing might build resentment long term. Though in the short term you both benefit.

But as I’m sure you’re aware, any money issues may sour the relationship. Even just having a formal contract with exchange of money could change the dynamic drastically.

Sounds like you’re well paid and your time is valued, I imagine most devs would be happy with that!