I never start a show I don't think I'll be interested in, so I can't say I've experienced this exact thing. But there have been a lot of shows that started out amazing and then went downhill, so I quit watching after a few episodes into whatever season "jumped the shark" for me.

The X-Files was my first. After Mulder left, there was no point in watching anymore. Then it was The Simpsons. After season 4 I was only watching out of habit, and then finally quit after season 8, and I have no idea why I lasted for 4 more seasons. I quit watching Family guy after Brian died. And all the other shows I have watched just end up getting canceled for no reason.

I think I'm getting to the point where I am waiting for a second or third season of a show to come out before I get into it anymore. Got other stuff to do than start a show that will end before it's time.