I think airlines of any significance explore all viable options in some detail. So if an operator sits down and thrashes out agreement on a few key matters, maybe carry out technical assesments, this only means the airport keeps itself in the frame as a possible - no more than that.
Of course, if you're talking about an airport where key slots are as rare as hen's teeth, the balance of power shifts. But SEN isn't in that catagory.
I think it's fair to say the Monarch situation hasn't helped SEN, neither has Brexit (some say). But I don't think you can conclude that SEN would have announced another major operator by now if those events had not happened.