How Much Fitness Do We Keep Year To Year?

Great Cat 4 Question:

When we go into a winter base/build phase
(of some sort, I know you're not a completely 100% traditional "winter
pure base miles" guy), to what extent are we building on top of the
season's gains, versus trying to limit losses? Should I expect to emerge
significantly stronger year after year of build phases, or to take big
dips and maybe emerge a little bit ahead each time, like this year's
power curve vs. last year's? How much does it all stick through a
stereotypical Z2 + gym winter? What is the time-scale on which all this
stuff plays out and how do you make power-building stick?

https://youtu.be/iKXNMzqZCF0