Hi BAlady,
The 58 seat figure for the F50 is the high density layout but it's more likely to be nearer 50 with VLM. I think the business case for the replacement would depend very much on the routes they plan on operating. If you assume they go for the same routes as now (incl. the same WX / VLM split), then I'd argue the ATR72 is the way to go. The direct operating cost difference between the ATR42 and ATR72 is minimal so the additional capacity of the 72 could win the day. I don't have figures available but I'd imagine the Q400 is more expensive to operate than an ATR72 based purely on the size of the aircraft but also on the engine SHP. However, if you now view the CityJet network in total (which hopefully they will do!!), then there is more scope for using the range and speed of the Q400 across some routes currently served by the RJ's. This could result in a drop in the number of jets in the fleet but result in an increase in turboprops.
Bombardier have drawn the curtain on the Q300, the last one rolled off the line last month and is destined for a maritime patrol role (if my memory serves me).
Air France / KLM don't appear to have a "joined up" approach to fleet planning yet so while the Regional and Cityhopper may operate the E-Jet, it isn't a forgone conclusion that WX would go for them.
As usual, these are just my two cents...
All the best,
P2C