Just my opinion:-
Its all well and good saying Baby are too slow to expand, but that is from an outsiders point of view. We can all speculate, but only the people that run baby know the real reason why there isn't a 4th aircraft yet. The press releases that they constantly issue state their dedication to CWL and its future prospects. Dedication means standing by something. Baby have stood by CWL as it is a profitable base. They have released new routes, BCN/WAW/GDK/MJV. They are testing the market with the resources that they currently have - 3 a/c.
The TCX A321 was based at CWL by MYT last year, before the final plans of the merger were released.
Highland are hardly dedicated to CWL. They are going along with the funding they receive from the Assembly. If highland were dedicated they would utilise their aircraft better, rather than having it parked up for 19 hours a day.
XL and Ryanair had their deals, then run, mainly to do with the fees CWL charge. Ask yourself, how dedicated is CWL to the airlines?
The above also has a link as to why TOM are pulling a 757.
There is still no confirmation as to who will operate SFB next year, as Thomas Cook Holidays still do not have it on sale. However i would love to see an airline like Monarch base an A320/321 at CWL, they seem to be going from strength to strength, but again concentrating on their main bases at the moment.
GSM on the other hand have a lot of work to do up in Scotland to rebuild their reputation before starting down south. Adding to that they now have a route from BRS, in competition with Zoom.
Zoom however seem to be making a success of YYZ with a twice weekly service and now year round.