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