If an Embraer 170 can get into London City, I would suspect it can also get into Paro. But in terms of size, that's no better than the BAe 146-100s that Druk uses right now. The RJX-85s that they had ordered would have given them greater capacity with greater range. They could get a second-hand RJ85 or RJ100 from someone, but that wouldn't give them the range to do non-stop flight to Bangkok, which is their key route. So there really is no replacement for the RJX-85s they had ordered. They can get the capability, but not the capacity; or they can get the capacity, but not the capability.
Druk are probably the most restricted operator, but there are others that face similar constraints on parts of their system. Air Wisconsin at Aspen. Meridiana at Florence. Crossair at Lugano. Malmo at Bromma. Plus all those carriers that get a premium from charter companies for flying pax direct to ski destinations (Chambery, Sion, etc) rather than a 3-hour bus ride away (Geneva, Lyon). Not a huge market, but it would have been enough to sustain the RJX had September 11 not wiped any possibility of orders from the table for the next 6-12 months.