I'm happy to say that I spoke a little too soon. Shotwell was finally picked up and taken over by Gnome and is very actively developed:
https://git.gnome.org/browse/shotwell/tree/NEWS Memory leak was fixed in 0.24.0. So it should definitely be updated, closing not only 839014 but also 777499, 806248, 688039, 824164 and probably several others.
