Most of the time our fixed bitrate cameras have enough bitrate, so there is nothing to be gained from more compression, it is only when they run out of bitrate that you will see differences.
Under trees at high speed it is easy to run out of bitrate, but the extra optimizations available in h265 are of no help in that situation, they just don't do what is needed.
In parking mode it is easy to run out of bitrate if there is a lot of movement, possibly of traffic, more likely from wind blowing trees about, in this situation H265 should be able to help. But we don't see this situation very often, the parking mode bitrate is selected to be more than enough for most parking situations.
So in real life H265 is of no real advantage for current dashcams, and variable bitrate cameras don't seem to be going to appear anytime soon.
As you have already pointed out, with current dashcams there is no significant difference in power consumption, so there is nothing to gain by doing the maths on it!
Taking this into account, what you're saying is as follows:
1. We're increasing the resolution (1080p --> 2k -> 4k, etc)
2. By increasing the resolution we decrease pixel size, decreasing the amount of light, which becomes more noticeable in low light situations.
3. And to top it off, we're fixing the bitrate. Granted, Dash Cameras are Fixed and Aren't moving around, but scenery, motion, etc benefit significant;y from variable bitrate. As a fixed bitrate doesn't adjust for these situations.
There have clips posted here, where a car passes by real fast, and the license plate is unreadable. A scenario where this comes into play. Especially as resolution increases, and light decreases.
So basically, ya h.x265 is useless, because current ARM chips cannot take advantage of h.x265's full potential. In this case, power consumption, compression, don't really make much a difference.
So why bother having h.x265?
@Nath, I'm fully aware that modern chips have h.x265 decoding built into the chipset. However, per the above, I was unaware that Dash Cameras used a fix bitrate. Otherwise, everything I said about CPU consumption, compression, and extra heat would be applicable.