The trouble is that existing CO2 detectors only detect a level of CO2 which, if it hasn't already killed you, is rapidly on its way to doing just that. It only takes a very small amount of CO2 in the inhalation side to seriously impair your functions. You're perfectly right about the harsh environment though - the lid of the scrubber (where you would put them) generally sits at 100% humidity but that can vary significantly, compounding the difficulty.
The temperature sensors measure where the reaction front of
a properly functioning scrubber is, ie from the bottom working its way to the top (to use the Evolution as an example). What they won't pick up is CO2 sneaking through the gaps in a badly packed or settled scrubber, nor CO2 that's bypassed the scrubber stack altogether in the case of a missing lid O-ring, nor a seriously 'overbreathed' scrubber.
I personally don't have a lot of time for the temp sensors. Call me an old luddite, but there are strict guidelines set down by APD about the life of the scrubber under various conditions - if you follow them & chuck it out when you're supposed to then you can't go far wrong. A bar graph that tells you you're 3/4 of the way through the scrubber when the manufacturer's limit has already passed is IMHO going to encourage nitwits to push the scrubber life. Sofnolime's cheap, funerals ain't
As regards cost. I for one would love to see them get cheaper, but the Insp is still the thick end of £4k + training after 6 yrs of manufacture & all the major CCRs similar money. The KISS is cheaper (and many would argue better) by virtue of its simplicity but it's still a ton of cash compared to what 99% of the world's population will pay for their diving kit. Dont even ASK what an RB80 costs, then again they're not really aimed at the recreational (sic) market.
It'd be good to see them become more mainstream, but to really take off they'll need to be
much cheaper, simpler, more robust, reliable, easier to look after and -dare I say it- less lethal when improperly used. I'm sure we've all seen manky old rental regs being used by first-time try divers, OW students & 7 dives-per-year-types at holiday resorts around the world. Treat a CCR with the same lack of respect & it'll bump you off.
As years go by, you're certainly going to see loads more around the place - the major mfrs are selling all they can build - but I reckon it'll be a good long while before they're as accessible as regular SCUBA.