Fundamental technical issues
Having spend the last 30 years working on mobile comms, this is an example of a government decision made by people who have no engineering knowledge, and a disdain for those who do.
They have taken a dedicated system, designed for reliability, specific function, and operating at a low frequency (which gives it wide range, good building penetration and requires a small number of base-stations), and are attempting to replace it with a "service" piggybacked on a commercial high frequency systems (so inherently poorer coverage, worse service indoors, and needing a massive number of base-stations to achieve the required universal coverage).
The underlying commercial infrastructure is understandably designed and optimized to maximize operator return on their spectrum and hardware investment _not_ emergency service availability. As such it is fundamentally incompatible with the desired operational characteristics of the emergency comms service.
The only way to make it work is by massively incentivizing the commercial operators to provide 100% cell coverage on their networks, and by overruling all the planning constraints which mean cell towers get rejected. Arguably 100% commercial cell coverage is a useful thing in its own right, but it would require a huge level of investment which makes no commercial sense, and which the government has never been willing to fund.