There is nothing wrong with the GUE approach, actually, there is quite some for it.
But aside from deep technical diving, where not everyone has sponsors, very deep pockets, and a great following willing to haul your gear to the site, there are those of us that just enjoy diving rebreathers. Me among them. I never had a single good reason to dive in the first place, other than making a childhood dream come true and enjoying it. Nor do I have any particular reason for diving a rebreather, other than enjoying it considerably more then OC (and making that childhood dream come true - turned out Hass and his wife were on rebrathers as the Aqualung hadn't been invented yet).
As for the timeline, I believe it'll be sooner than 50, 40 or 30 years that rebreathers will be common place. Aqualung has been building rebreathers for years for the military, Oceanic wanted to get into it 10 years ago, and just bought the rights to the PRISM from Steam Machines. Poseidon teamed up with Cis-Lunar. As with anything else, once the big companies smell profits, they'll want part of it. That'll lead to investments badly needed in this sector of the market, it will bring new technology and most likely more safety. The innovations of Bill Stone and his design team, combined with the engineering expertise of Poseidon is but a hint of what's to come.
The CO2 sensing technology for rebreathers is available, has been for a few years, once a high volume manufacturer is willing to pay for it the unit cost will come to a reasonable level.
The two main reasons that people seem to list here are expense and safety.
The price is in numbers, at least half the units out there are made from machined parts and hand assembled. The rest are made from molded parts and hand assembled, but still in fairly limited numbers, making offsetting the molds rather expensive. Imagine your masks being completely made by hand ... or look at the price of Force Fins. Low volume, handcrafted items cost money.
Safety is and will continue improving. Higher quality control, more thorough testing, new and improved technology. The eCCR is just 40 years old, and has been a niche market. Think were cars were in the 40s ... in the 70s ... and where they are today. It's still early on for eCCRs, the first volume model (call it the Ford Model T of rebreathers, available in any color as long as it's yellow

) is just 10 years old this year. Technology is moving faster these days, and with bigger money there'll be bigger advancements.
The Poseidon Cis-Lunar MK6 Discovery is an incredibly gutsy project, if it succeeds it will be a big advancement towards user friendliness and safety. If they can keep the price down, even a step towards affordability.