Bernard Gustin said in an interview to VTM that the RJ100 is quite old and has 4 engines, so not that cheap in costs compared to other types of aircraft. If you say that, this is a contradiction with the fact that you'll be flying in Germany with old 4-engined A343's. Shows quite clearly that LH decided the routes at DUS for SN...
A340 were produced till 2011, but from late 1990s to mid 2000s, they get some improvements with the A340-300X and A340-300E versions. Improvements are Engines, avionics, MTWO (maximum take off weight) and Cabin, based mainly on A330 for the A340X and mainly on the A340-500 / 600, for the A340-300E
Air France (and Joon), Swiss (and Edelweiss Airlines) and Lufthansa (and Lufthansa CityLine) use only A340X or A340E. Early built (1992-1995) A340-200s / 300s were phased at Lufthansa and Air France
Airlines look also the number of flying hours and number of cycles (take off and landing). Traffic is growing at Brussels Airlines and i think more and more routes are too big to be served with Avro, not like years ago, when the airline was struggling.
Remaining tinies routes are now served by BMI or CityJet with SSJ and it seems that SN is quite happy. Maybe, the airline will order regional aircraft in the future, but i think it will depend of Lufthansa projects.