Well, if you look at the Enterprise's example from Star Trek II, becoming a training vessel is a viable solution. Remember, she's about 40 years old at that point, and apparently still has some service life in her (since she's supposedly getting a new, younger crew after the cadet cruise is over).
Now, looking into TNG, we find that the Excelsior and Miranda classes have ships that actively serve for 60+ years. Starfleet probably had many more that act as training vessels. And post-DW is a bad example, since the fleet will probably need every ship it can field following wartime losses, and older ships will be upgraded to stay as up-to-date as possible. There probably won't be a mass decommissioning of older ships until at least the 2385-2390 timeframe, and they would be so old that they would have little use for other purposes.
Considering your question on a Daedelus in TOS, I'd say that there are probably only a few surviving ships...one or two are now museums, another one may be a training vessel for a planetary defense force (although I would expect that most PDFs would have ships more up-to-date than a century-old model).
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz