United doesn't, as far as I can tell, do maintenance until something breaks.
In order to be allowed to fly in the US, and other countries, you better believe United does maintenance.
This is one of my favorite reddit comments ever, and goes over aircraft maintenance from a car analogy, explaining what you'd have to do to a car in order to meet the mandatory maintenance standards imposed on airlines as a matter of law:
https://www.reddit.com/r/aviation/comments/3v1hzj/how_do_old...