I know you could get a dud, but this doesn't seem like an issue at all. Decades? C'mon son!
I kid you not. It does depend on the amp design and how it’s driven.
People will change tubes in a gigging amp as part of a regular maintenance program. Whether or not they need to. Some will install new tubes before going out on tour or into the studio. But people do tend to change tubes more often than they need to.
One thing that compares are the flats on my basses. I never change them unless I want to try something else for a few days. But then the old ones always go back on.