A few months ago I had the same exact thoughts as you. For some reason tho, it didn't stick. At first I thought, "well I guess OW is just better than Gigantic after all," but then I switched back to Gigantic, and so on... Gigantic has ended up being what I play when I'm (temporarily) burnt-out on Overwatch, rather than the other way around. Took me a while to figure out why, but after a few cycles of that, I realized it was OW's simplicity that kept me coming back. For better or worse, OW has fewer "mechanics" like all the different statuses, doesn't have skill trees, and on top of that the game doesn't communicate those mechanics to you as clearly (by virtue of there being many of them, I think). In OW, there are recognizable ability callouts, visual effects for statuses and damage types... the game just communicates in a simpler, clearer way. To me, at least. Gigantic revels in the chaos of all those mechanics, the frenzied fights, multiple simultaneous team goals. Don't get me wrong, I absolutely love Gigantic. Everything that makes it different from OW doesn't make it Lesser, just... well Different. But it's enough Different that I just get burnt out quicker playing Gigantic.
I'll get off my soapbox in a sec, but just know that I don't think these two games should necessarily be replacements of each other. They're similar, but also different. Let each fill a unique niche in your gaming life.