KLM seem to have a nice business at NCL with (in my experience) pretty full 737s flying to AMS. If they deliberately set out to move the Teesside flights (which for all any of us know could well making a lot of money for KLM) to NCL they would have to consider:
a) How many customers would just use Easyjet to fly to AMS?
b) How many would change carriers and transfer through LHR instead of AMS?
c) How much would the load factor on the KL 737s from NCL increase with no flights from Teesside?
d) What would be the lost revenue on NCL flights (prices are normally c£100 more from Teesside)?
e) How much would transfer to LBA (Jet2 compete with KL on this route)?
I must have taken 30 flights out of Teesside in the past 18 months and in my experience the morning and evening flights are pretty full, even with the prices that KL charge. The airport may be losing a fortune but I don't think KL are on this route.