I only saw Jack in-world once and witnessed a reasonably long chat between he and Prokofy Neva. I will say that Jack certainly had a great sense of humour, even while joking Prok wasn't easy on him. There are those that seem to want to blame Jack for much of the failures of these last three years... I don't know. I would suspect that he was an employee, and as such he answered to a supervisor. Did he make decisions that didn't pan out well...likely. It would be the height of folly to imagine that everyone makes the right decision every time. For a quick list of Jack Lindens contributions see Prokofys Second Thoughts blog.
I have mixed emotions about Jack leaving the Lab, the continuity provided by The Old Guard is comforting at some level... but I'm trying to balance that comfort with the need for new people with a new view. What do you think?
And so it goes
My love to you all, brinda