You're right, recovery rides can serve as mental sponges, soaking up lessons from high-intensity workouts. But let's not forget that they're still just "recovery" rides – their main purpose is to help your body recover and adapt to the intense efforts.
As for your question about their impact on sustainable gains, well, it's a bit of a gray area. Some studies suggest that active recovery can enhance performance and adaptation, but the effects are highly individual and can depend on a variety of factors.
Personally, I've found that incorporating regular recovery rides into my training has helped me feel fresher and more energized for my high-intensity sessions. But I still believe that the real gains come from those short, intense workouts you mentioned.
If you're looking to compensate for a lack of endurance, I'd recommend focusing on sweet spot training – intense efforts at or near your FTP that challenge your aerobic system without completely annihilating you. You can adjust the FTP builder to include more of these efforts, or even create custom workouts that focus specifically on sweet spot training.
As for results, I've seen significant improvements in my own fitness and performance by incorporating more sweet spot work into my training. But of course, your mileage may vary. Give it a try and see how it works for you!