Not every kids is like your son. Some have more addictive tendencies and wont self regulate that easily and wont stick to agreements - even as they had an intention to follow up in advance.
Well sure but the ability to self regulate doesn't come out of nowhere. It's learned behaviour and xbox or a video game is a relatively harmless thing to train them on because the consequences for failure are minimal. That they're going to fuck up occasionally or even a lot isn't good reason to not keep trying to train them on it.
It does not come from nowhere. And for many kids it simply does not appear just because you threatened to remove xbox. And followed up on that threst.
The poor impulse control remains poor impulse control. They will grow up from that, but in the mean time if you expect self regulation they are not ready for yet, both you and them are in for very unhappy time when they both play too much and are regularly punished for that - despite not being capable to self regulate yet. One would almost say that it is unfair.