I've thought about it and I do think it is a phase thing for most people. The thing is, with a band like Opeth, unlike someone you're embarassed about liking long after you've stopped listening, you'll sort of phase in and out over time. Sure, I've stopped listening to Opeth a lot less, and at this point in time they are no longer my favourite band, but maybe in a few months or years I will have another long phase of listening to them, and it is very likely they will be my favourite band again. I doubt any of you will say that Opeth have been your favourite, most played band for over 5 years or whatever (for the newer fans it's easier to say they are still your favourite).
At the moment Led Zeppelin are my favourite band, just like they were about 7 years ago, and they might be again in another 7 years. The situation is the same with Opeth