Loganair was not going to operate CWL-GLA; it was going to operate CWL-EDI.
In early December last year Flybe announced the withdrawal of its CWL-GLA and CWL-CDG routes from 19 January this year.
On 11 December last year Loganair announced it would take over Flybe's CWL-EDI route as the latter was going to withdraw it at the end of March.
Press Office :: Welsh route for Loganair - Loganair
CityJet then came on the scene announcing it would take over the CWL-GLA and CWL-Paris routes from January (but to ORY, not CDG) as well as CWL-EDI from the end of March and CWL-JER in summer (actually in competition with Flybe to JER which is what happened).
With Loganair this would have meant up to 6 daily flights each way between CWL and EDI. Shortly afterwards Loganair cancelled its proposed CWL-EDI route before it had begun leaving the field clear for CityJet.
Re the question about fares, anecdotal evidence in local aviation websites suggests that the CityJet fares were/are relatively high leading some people to travel to Bristol for easyJet flights (on both the EDI and GLA routes).
It does appear that possible replacements could probably only come from Eastern or Loganair, unless Flybe itself has a change of heart (unlikely I would have thought and if they did they'd surely want EDI as well and what would that do to CityJet at CWL if they were of a mind to stick around?). Ryanair's aircraft are almost certainly too large. bmi baby's smaller (than Ryanair's) 737s turned out to be too big for the route and within a year or two they had engaged Air Wales with ATR 42s to operate for them, and switched to PIK from GLA.
A real outsider might be bmi regional. They have five based aircraft at BRS with persistent rumours locally that the base there will expand possibly as soon as next year.