I have looked at the research on CD and it is mostly about increases in core temperature. So what would change core temperature? Ambient temperature, body fat, clothing, air movement and CO2 concentration v O2. When I am indoors, ambient temperature and fan air flow rate dominate my cardiac drift. If I am skinnier then I get better heat regulation and lower CD. How about a closed room full of cyclists consuming O2 and breathing out CO2? Not good. Indoor cycling requires good ventilation of fresh air otherwise there is too much CO2 and that is our nemesis in power production...getting rid of CO2. Outdoors, we are constantly bathed in air that has constant O2/CO2 ratio. Hence the trainer is harder and CD an interesting variable that may have tertiary value at BEST. I consider HR in general a tertiary measure with too many unknowns to provide a lot of benefit. Relying on a derivative of a derivative seems dumb.
For a moment, let me argue the other side. CD is the new new thing in training and everyone should be indoors doing 120 minute workouts until CD is X. Assuming that is true, a workout is only of value if an athlete is willing to do it. I am not interested in 120 minute workouts on a trainer monitoring my HR. That is a non starter and I will do something else.
I do indoor workouts only when I have to and only when I am highly motivated. Also, I know that indoor workouts increase my mental fatigue more than the same TSS of an outdoor workout. So why would I do them? If I only wanted to ride inside to increase fitness, I would do a spin class, Zwift or Peloton and use their software. If I go to spin class, all I need is clothes and shoes.