OK, I'll bite: how long is "a while"? I've seen other posts that say even 7 days off the bike will show a significant change in muscle composition. ...at what point during the winter do you say, "I've lost all/most the progress I made last year, so my Spring training needs to be all about building a new base".
A powermeter would let you quantify your fitness at any time in the season.
A winter of long easy rides will wipe out a lot of your fitness. With a powermeter, you'd be able to seen big declines in power for almost all durations indicating all energy systems are detraining. Goodbye mitochondria, goodbye capillaries.
