...Until they switch to a bandwidth exhaustion attack.
There's no argument that a private line is ideal for critical infrastructure, but if they must make do, there are ways to make it work.
Anyone not behind one of the major CDNs can be targeted and taken down. And bandwidth-exhausting a few routers in front of the target is certainly possible, it's just not profitable for cybercriminals - but for a nation-state? No problem at all to muster a few terabit/s of capacity for them.