Have you every looked at either the USL or Linux source tree?
Are you familiar with the RCU functionality that IBM (nee Sequent)
donated to Linux which was the primary functionality subject to
clean-rooming it away isn't so easy as you imagine.
Frankly, the lawsuit was bogus from the start. But once bad lawyers
gets their teeth into something, all bets are off, since they get
paid regardless of the outcome.
And had they won, there would be a large payout and the revenue
stream would continue as long as one Redhat or Suse customer
was still using releases with the infringing technology.