As a 47 year old mountain biker, I fully agree with this. During the winter I did SBMV1 followed by SSBHV1 likely with an overestimated FTP. Then I went into Sustained Build MV & by the 2nd week, the wheels were coming off the cart. I dug myself into a deep deep hole physically for sure but maybe even moreso, psychologically. I ended up finishing Sustained Build in Low Volume & did Low Volume Marathon MTB for specialty & rode outside a bunch as the weather got nicer.
When things were going bad, it was a nightmare. I’d get on the trainer & 1/2 way into a workout I’d be a wreck as I couldn’t do them even close to 100% & even when I could push hard physically, my mind simply did not want to do it. I’m not a quitter but I had to stop more than a handful of workouts which threw me for a loop. Once I dropped to low volume & got back to fun riding out side, things went well.
Since April, I’ve been doing low volume plans but only 2 days a week,usually skipping the 1.5 hour indoor session & doing 7-9 additional hours of riding how I want outside. It’s been working great for me. My FTP is steadily increasing & I’m getting faster, posting PR’s I’ve had since 2015. So to the OP, don’t be afraid to drop volume or tweak the plans to work around your fun riding outside. I bet you’ll still get benefits from the plans & get faster as a result.