Watching this thread, this was my feeling also: base only lasts so long. My experience is that I put on base in winter and spring, then in summer I get into doing very hard long rides and events almost every weekend. It takes me so long to recover from these that my base gradually deteriorates and by August my endurance is not so good.

Since common practice is to do base in winter, it makes me think that base isn't eternal: it only lasts for a few months. "Base" meaning that fat burning effect. We can go along for quite a while without doing any base, but the middle of the season is a heckuva poor time to find out that we've lost endurance.