Weirdly, there was already a good GNOME photo app written in Mono since the mid 2000s (F-Spot, originally by Ximian), though I believe gThumb remained the default in most distros until it began to be replaced by Shotwell at some point in the 2010s.

I don't agree with everything Ubuntu did in that time period but shooting down the idea of creating yet another competing app, rather than improving one of the existing options, sounds fairly reasonable to me.