In practice the vast majority of designers are using Mac, so it's not like it not being cross platform was the problem.
The thing that made Sketch lose against Figma was that they never really tackled the issue of collaboration until Figma came along, leaving it for 3rd parties like Invision and others to solve.
Now they have most of the collaboration features they were missing, but by then it was too late and Figma had all the mindshare.
But if Figma keeps following this route, who knows. We might see Sketch grow in mindshare again.
The thing that made Sketch lose against Figma was that they never really tackled the issue of collaboration until Figma came along, leaving it for 3rd parties like Invision and others to solve.
Now they have most of the collaboration features they were missing, but by then it was too late and Figma had all the mindshare.
But if Figma keeps following this route, who knows. We might see Sketch grow in mindshare again.