The back shell isn't really involved in heat dissipation per se. Most of the heat is carried by convection out the vent. The critical thing is that the back cover is carefully engineered, in coordination with the spacing to various bits of the internals, to selectively restrict airflow, to produce the needed flow patterns to cool all the internals.

But, yes: if you change the inner mould line there's the risk of some components not getting the airflow they need. It might not even be obvious, as not everything has a temperature sensor.

Apart from the Engineering tolerances of the backplate, The fan is synced to APU temperature and if a backplate aggressively cools the APU then the memory and other components wouldn't get cooled.

Then again the eventual 'Water-cooled Steam Deck' modders would know about this already(Water-cooled Xbox/PS have very high memory chip failures due to improper cooling).

I'd love a replacement shell with threaded screw holes so that, when the joysticks eventually drift / battery starts to die, I can feel safe opening/replacing things.

That makes sense, Opening the shell seems to be bit finicky and I hope the plastic clips are strong enough.

But TBH, A compute hardware which is designed to be opened from a mainstream company is something I never thought would see again. I really hope general consumers dig into this and demand it from others.