Disagree it was dragging on and on, he wanted out and the club also wanted him out so they could get their man and give him at least some input to the summer transfer business. If they recruited a new manager while he was still there it could well have been grounds for constructive dismissal from Russel and us having to pay up his contract rather than vice versa. It should have gone to a tribunal but why that hasn't happened is anyone's guess.
Ultimately it looks like we have now gone after RM himself rather than Southampton (or possibly in addition to since it is suing for breach of contract).