And to follow on from
arfortune's post, see
here, which refers to the steep approach modification being a software mod.
I don't think that there would be any significant technical obstacle to KLM operating E190s into LCY if they chose to do so. Rostering flight crew to make sure they stay current on LCY ops would be as big an issue (and of course this can be solved too).
In my view, KLM would be unlikely to look externally for an AMS-LCY solution when they have the ability to serve this route internally through Cityhopper (and where they can potentially use the offer of the route to Cityhopper as a sweetener in other negotiations with their pilots).
But to return to the subject of this thread, Cityjet...
I believe that as part of the sale of Cityjet, a codeshare agreement with AF/KL was put in place for a period of (I suspect) 3 years, so Cityjet can at least rely on benefitting from the KL code on routes like LCY-AMS for that long.
(That's if any profits from LCY-AMS are sufficient to compensate for the cash drain from the LCY-DUB bloodbath.
)