As I recall reading a while back, they kept raising the term to retirement because the available supply of farmland for soldiers became scarce; much like the same strategy we’re using by increasing social security eligibility age.
Relatively few of the provinces generated surplus revenue for the Empire as a whole, but Britain was pretty nice, at least one Roman writer remarked on how blessed its climate was, and at some point a big canal was built to both drain new farmland and to provide transport. This is despite Britain's not quite supporting the cost of the garrison, which wasn't all that large btw. Villas abounded, and those are easier to find, so it's likely that there 100s of 1000s of undetected farms, mostly from locals, Romanized locals, and Roman pensioners.