The ports help to keep people interested in the Switch while Nintendo are working on new games. I doubt Nintendo are anywhere near done releasing ports (whether it's from 3DS, Wii U or other consoles). So we could see more of an influx of releases in the coming few years.
I think Nintendo are trying to get what little bit more they can out of the 3DS while it's still somewhat relevant though.
I don't understand how games I already bought on the Wii U are supposed to keep me interested tbh.