The aircraft has to have mods to allow it above FL280: RVSM is one but I recall there are others.
The RJ only needed an ADC update for RVSM. I would be surprised if any operators had not taken this up as it was a simple box swap.
Most 146s do not have RVSM capability - a few do but as the maximum 146 altitude is 30 or 31000ft then the RVSM mod has not been taken up by many operators.
The Fokker might cruise higher than a 146 but not an RJ. I suspect the Fokker short field performance is lacking in some areas compared to the 146/RJ otherwise they would operate into LCY - and they can't.
If they cannot be operated at profit, I wonder why Lufthansa, Brussels, Swiss and TNT have operated such large fleets for as long?