Surely high speed trains suffer from network wide monitoring outages too? I would imagine they're susceptable to similar issues.

Yes - see German trains going down for that reason a few months back:

https://www.reddit.com/r/germany/comments/1udup2j/trains_hal...

Sure, but having a _second_ way to get places can only help.

still, it would be nice to have an alternative as a backup.

But likely not at the same time as the planes.