> maybe someone can shed some light into the downsides of Svelte
Svelte trades off runtime size for component size. It was created in the context of infographics for the New York Times online and for projects that roughly line up with that it's pretty much the technically best option.
I like the Svelte authoring experience and introduced it for a few components in a React based low-code platform. The reason I phased it out was a chat component that was ~600 LoC and 50-something reactive variables in a moderately complex chain blew up into ~5k LoC of output. I also ran into what seemed to be some transient invalidation issues. I was short on time to debug this and engage with the Svelte community so I rewrote it in React to match the rest of the system. It's possible I was doing something wrong but I don't have enough confidence to bet on 3.x again. I'll take another look when 4.x comes around.
> Presumably if Svelte were the be-all-end-all of front-end frameworks, it would dominate soon enough?
There's significant network effects around the established frameworks. Nobody gets fired for picking React. I personally think Solid is the best overall technically but the ecosystem and mindshare is smaller and that matters to a company making a business and not necessarily technical decision.