KLM vs Jet2
The experience at LBA was that the KLM passenger figures stayed almost constant (or even increased in some months) after Jet2 started their LBA-AMS route. The 10-11,000 pax per month using Jet2 were all "new" pax for the route. I have no information on the fares being charged by KLM before vs after Jet2, to judge the yield level, but they don't seem to be drastically lower than pre-Jet2.
I don't know what the effect of Jet2 on KLM and MAN has been but they should know by now if they can co-exist or not, and in the case of the LBA the answer seems to be Yes.