Shouldn't an ISP be responsible like many have been since the 90's and trunk you to a backup server during maintenance to keep your service uniterupted.
Backup? For residential service? Since when? I've had several ISPs from various DSL providers, HughesNet satellite, and even Comcast and I've never been afforded that luxury.
You can always subscribe to Comcast Business Class service if you need a constant connection. Or get a secondary ISP.
FWIW, their Business Class Service is delivered on exactly the same infrastructure as residential service. If hardware / lines / plant go down, both services go down. Same thing if a vehicle takes out a pole.