I never understood the ecosystem argument as applied to Svelte. Vanilla js libraries integrate with it pretty much perfectly, meaning you don't need a Svelte port of a given library. The ecosystem is effectively huge.
I've rarely used React, are you talking about React-specific libraries? I find it hard to believe that there are more maintained React libraries than vanilla JS libraries.
It basically singlehandedly convinced me that the ecosystem was worth going with react when I started my project, despite really enjoying svelte.