For me, the biggest downside to Svelte is the lack of TypeScript support. This is a massive deal-breaker for myself as well as many other developers. I am also not a fan of the Moustache/Handlebars inspired templating, it feels outdated in the face of enhancing standard HTML or going in the direction of something more extreme like JSX.

I think Svelte is refreshing, but I would not be comfortable using it on a large-scale web application just yet given the downsides, particularly an enterprise LOB application with many moving pieces. This is why frameworks like Angular, Ember and Aurelia on the front-end lend themselves better to the enterprise and React and other more component-based libraries do not.

In terms of community size, I don't think that matters in 2020. You don't need a big community if you have something straightforward and easy to work with. Great read.