Training FTP is one of the least demanding uses for a power meter. That's why riders have been able to train effectively with nothing more than a regular route and a wristwatch or a speedometer. An indoor trainer is like a regular route so if you have a wristwatch and a speedometer you can train effectively. Trainerroad started off doing trainer plans with dumb trainers, and riders got better.

There are other, more demanding, uses for a power meter, but most riders use them as if they were fancier HRMs.

i understand the point about not getting a power meter if you enjoy riding your bike. it can become the boss for those who aren't careful; but I'm with you on this - the improvement and even enjoyment it brings on the hard days are worth it.

i do sometimes kike to ride my cross bike or my winter bike and leave the pm at home - but that us my problem with self-discipline and something to work on.

Along those lines, for the last three months I have been cycling almost entirely outdoors only, vs. indoors M-F and outdoors on the weekend (weather permitting). I have not done this in decades, and it has indeed been quite pleasurable. There has been no structure to my riding, though, and no hills or others around to push me. So, I have been JRA at what ends up being essentially the upper limit of polarized zone 1. Despite doubling my overall volume, when forced inside by rain earlier this week, it is clear that I have actually lost a bit of fitness. All of which brings to mind the PPP:

"Sooner or later, you have to increase the power"