In my experience, a lot of places that want to do redundant wired Internet have the local fiber provider and local cable provider going into some failover or SD

In my experience, a lot of places that want to do redundant wired Internet have the local fiber provider and local cable provider going into some failover or SD-WAN setup. T1 would be rare (but possible) for Internet access, not rare for phone (funny thing is the phone might be SIP over Internet over T1). You might even have crappy situations due to people not understanding what they are buying (or other weird reasons) where 2 ISPs share 1 LEC circuit, e.g. a single physical fiber line. But anyway, a provider cutting you off permanently is more than an outage. You need to get another ISP.