I read an article today that states your first sentence. There is some evidence that omicron is showing that Covid is mutating towards a common cold type virus. Which is what some experts predicted months ago could happen. It is a coronavirus after all, which causes 25% of the common colds. So, it would be no surprise if Covid eventually heads in that direction, is the theory.
Of course, the article did say it's way too early to know either way, and whether omicron is the mutation that even starts us down that path.
In the meantime, I got my booster last week.
Natural Selection dictates that viruses mutate into more infectious forms (as the less infectious forms get crowded out, simmilar to how Delta effectively ended all other variants to date). Severity is neither beneficial nor really detrimental, but increased infectiousness is most certainly beneficial to the virus. If more infectiousness happens to come with less severity, then the virus will become more present but less severe. But the severity is virtually irrelevant to evolution (unless it magically mutates into a Captain Tripps-level doomsday thing). The sole purpose of a virus is to replicate. Anything that doesn't increase replication is tangtiential.
Practically speaking, if Omicron is more infectious than Delta, it will likely win out. Its severity is irrelevant to that.