Greenbike, I don’t think anyone is going to make 10 hours per week of sweetspot. 
If you have ten hours per week, you can easily do two sessions of intervals with easy endurance riding the rest of the time and get plenty fit.
That said, you can use sweetspot to increase training load and build durability.
I used the principles of coach Steve Neal and hit my best numbers in recent years. And honestly my FTP only went up maybe 30 watts during my build cycle but I felt very strong on the bike (stronger than ever).
Coach Neal’s methods seem to count on increasing training load and building out TTE (time to exhaustion). In my case I was doing two sessions per week and built out my sweet spot to 4x20 minutes. (Nothing crazy as you hear about people doing more.)
For me 4x20 was plenty as this took about 2 hours on the trainer with a warm up and cool down and was the limit of my butt’s ability to stay on a trainer.