Problem is, the device doesn't have to know, which address is the working one - e.g. which prefix is working on the upstream. It might break connections etc. A solution of course could be to have NAT66 and I really mean address translation not masquerading/ port translation here - as such, much easier to do as it is a 1:1 mapping or to get provider independent space or become a LIR and have two providers announce your space with respective BGP configuration for failover. It is a solvable problem but isn't much guidance and experience with this at smaller companies that might not want to run BGP or go through hoops to get PI space etc.

Exactly this.

And you are right BGP is a non-starter for smaller sites, especially where cellular is involved. Often small site businesses just need a backup internet connection to access the web and emails, which doesn’t need the overhead of BGP. Also there is a non-trivial cost to using BGP (equipment, expertise, and ISP will charge more for the privilege) which for smaller business doesn’t make economic sense.