That's a remarkable situation for SN to be in - to wind back fleet after such a big event of retiring the RJs.
I was on BRS-BRU evening flight recently and it was 45/48 pax, so almost fully loaded..
I guess if SN are going to go in the EW direction, that means consistently offering easyJet level fares, and that does mean having lower cost per passenger seat kilometer, and I guess more inventory. That could be the change driver.(although still not sure that the RJ is lower cost per seat than the Emby, but I could be wrong).
It does also give real-life data point to BM's consistent claims to be looking at larger aircraft but it never quite materialising. If they couldn't justify a larger aircraft in this situation, I'd be surprised if it is going to be driven by other routes.