The operation is not over
Whilst there may be a whole host of other companies to step in to take over this lost business and, yes, SolarWinds may have screwed up with some fundamentals OpSec, I'm not sure suing them out of existence is going to do the world any good.
There would have likely been a reason many of these huge customers chose to use them and not some alternative. At a guess, maybe because they tick more boxes than the competition making them the "best".
Whilst there definitely should be a review of what happened (and i willingly prepare to eat my words), they were the target of a state level attack, there's very little they could probably do to have prevented the attack. What should be happening is learning lessons to make sure that a repeat can't ever happen again and to move on.
To use analogy, it would be like West Bromwich Albion firing Sam Johnstone (according to stats, he is best goalkeeper in Premier league) because they lost 4-0 to Arsenal (also Google it yourself). Maybe, there was nothing in his power he could have done to be better Arsenal were happily dancing around a laughable defense. The team itself should reflect on the situation, not just blame the goaly