For every rule there will be exceptions...

I was UA Premier for several years and then quit flying UA. They left me as Premier as long as I had a corporate address. As soon as I changed my address I was dropped, which didn't bother me since I haven't looked back since.

As FT mantra goes: YMMV, but if you are just an ordinary traveler (are there any of those?) then expect to drop one level the year following not making status.