There is no guarantee whatsoever that the computers would be restored if they paid the ransom.
Yes, and even if the crooks did release the systems, they could have left viruses hidden in obscure places to wreak untold havoc or repeat the whole episode monthly. I assume the hacker ransoms are priced low enough to undercut the cost of replacing every conceivable thing that could be infected. But with a pipeline like this, the cost of NOT replacing everything could be enormous. I see no way the operator could justify paying the ransom and keeping the vulnerability.
A big concern that will remain is that this was an inside job. I wonder if all employees and contractors have had to undergo lie detector tests.