As I've said before, LTN seems a very strange airport for FR. They generally fly the routes from LTN that for whatever reason are not served from STN and routes that I tend to think of as "traditional" FR routes such as DUB, NOC, KIR, GRO, REU, MJV etc. In other words, routes to airports that have been on the FR route map for a very long time.
LTN looks a bit like a typical FR base 10 years ago. It hasn't really updated or evolved into the type of network FR would operate now, which from the UK mainly includes ALC, AGP, PMI, FAO and typically a number of Polish and Italian cities.
Given these points, its most likely a case that LTN is not somewhere they wish to expand (probably due to proximity to STN and large EZY presence) but the routes they do operate are profitable, so would make little sense to get rid of them. I think "stagnant" will probably remain the best way to describe FR's presence at LTN going forward.