There is a retention policy, so the raw events aren't kept very long. The rollups probably compress really well in their time series database, which I'm guessing also has a retention policy.

If you have high cardinality metrics, it can still be really painful, although I think you will feel the pain initially and it won't take years. Usually these systems have a way to inspect what metrics or counters are using the most resources and then they can be reduced or purged from time to time.