I think as much as i like Msgplus Patchou you are indeed correct that it would be better to stop developing on a high note instead of adding useless features as MSN Messenger becomes a better more featured product.
I think you've reached a point where Msgplus has most features that people need? Unless some other people start suggesting features that everyone would like (elsewise it could be done as a plugin)?
Plus i think it would be good for you to start developing other projects which you are interested in
. You can always come back to msgplus for compatibility updates and adding minor features/ correcting errors.
But overall its your choice
, you'll have to see what the users of msgplus think and any possible features a majority of people would find useful - possibly another thread on this?
If you decided to do this i wouldn't even necessarily call this "finishing development" as you'd still be releasing compatibility updates and possibly fixing minor bugs/minor improvements - you'd just be spending less time on Msgplus.