I also would say, for this, I would try ERG mode at 20% or something. Really HR mode is a convenience of just forgetting about it and relaxing… If you are doing drills/skills work you can always use ERG mode and press ± to keep yourself in HR range.
I am not saying I won’t go lower than 40% in the future, I am just saying this seems like a very rare and isolated case and most people should be fine with either ERG, SLOPE or HR(with a typical cadence). If I am wrong, then I am wrong 
I think you’re right. No one should see the HR mode as a replacement for the ERG/Slope/Resistance modes. It’s a supplemental tool with it’s specific use case. Longer steady work at lower intensities.
Alex, no worry. I think you are correct. Just saying that trainer day can push a 40 watt’s resistance to my trainer.
I will just lower my cadence. To tell you the truth because we pick 65% of max hr, there is still some margin. My hr does not go so high that is in zone 4. Maybe beginning of zone 3. Thanks for all the advice.