It's funny how the successes and failures of these two companies ultimately comes down to a single architectural decision that both took different paths on. Sketch's biggest drawback (even back in the 2010's when they were on top) was always that they didn't support Web, Windows or Linux and focused only on Mac's. Honestly it paved the way for Figma to just come up and eat their lunch.
The biggest mistake that Sketch made was not realizing there was a sea change sooner and shifting their focus to a web based app or even releasing Windows and Linux clients. Even now I went to their website and they only offer basic viewing tools on the web, if you want to create with Sketch you need a Mac and there's no way around that.