Ensuring there isn't a single point of failure is not just as simple as 'putting it in the contract.' Carriers can and have groomed their primary and backup circuit on to the same L1 path without realizing it.
I've never seen it happen in 20 years. Although of course small parts of the path change over time and sometimes that means bits get put in the same duct for a short run but that also happens when you buy from different providers to be honest. With some carriers you can insist they send you updated route kmls when they change either path and you can detect the change as you'll see different losses/otdr traces before and after a maintenance/fix.
Meanwhile diverse ISPs wouldn't be using the same A and B. ISP A has a central office in a high rise downtown, ISP B primarily targets single family homes and has their switching equipment on a piece of land next to a substation in suburbia, so if you subscribe to both you not only have links coming from opposite directions, they're each operated by independent organizations instead of a monoculture.
In the last decade a number of ISPs have popped up and decided to fibre up areas. They are invariably buying OLTs from Nokia or Adtran (the same two vendors as BT OR) putting them in a BT OR exchange because that is cheap and very convenient, good access to backhaul etc and then renting BT OR ducts and poles to install the fibre in/on (PIA). To top it off they often are using the same fibre vendor as BT and sometimes even the same contractors to install it. Worked example of this; netomia/youfibre (though there are dozens).
What resilience are we really gaining here? Organisational, and that's pretty much it.
Now suppose that anyone could run an ISP out of their house. They wire up a few of their neighbors and then make a single long-distance run to one of many backhaul providers, none of which has a dominant market position like BT. The backhaul providers connect to their customers and each other in telco hotels, but they're smaller and more numerous because each of the largest providers has their own, so the city has three or four instead of one.
Meanwhile even if you have a BT, organizational independence in itself is better than nothing.
It is a fine concept but you'd have to actually ban infrastructure sharing between ISPs and be building a completely Greenfield network. Quite theoretical. I'm aware of former eastern bloc countries that have dynamics somewhat reminiscent of what you describe though.
You could require them to provide those services for their competitors at regulated rates, but then you're not actually breaking that monopoly, you're just regulating it while cementing it in place.
If you have an existing monopolist then first you have to thoroughly break them up, not just mitigate the continued existence of the monopoly.