I retired my first themes back in 2008 for a few different reasons. Surprisingly, I didn't get too much hate mail. For the most part, there was good feedback. Those themes were some of the most popular back in the day.

The important bit is to be completely transparent and have a plan of action to support current users for the rest of their membership terms. I wouldn't retire themes without bringing some new stuff in either..

Any theme shop knows that it's not a good idea to support and update old themes indefinitely unless they're still bringing in money. The only realistic way to do that is to have fees (monthly, yearly, etc.) for continuing the updates. Otherwise, you've got to drop off some of the dead weight every so often.

A good three-year-plan might be worth considering for theme shops. I think three years is a reasonable amount of time keep a theme going. Some might want to do two years or four years. You also have to take it on a theme-by-theme basis. There's a lot that goes into the decision to retire a theme.