What, like a couple more weeks? I find it hard to believe that an experienced React/Vue/Angular dev would take a long time to pick up Svelte.
For a junior, sure, but I would expect a mid-to-senior developer working on front-end JS for a living to have some experience of at least one of the popular rendering libraries/frameworks and therefore to be able to understand at least the essentials of any of the others after a few hours of training. They're relatively simple interfaces, and many of the same concepts come up in most or all of them. Of course it will take a bit longer to understand the edge cases, any unusual or unique aspects, and any common traps to avoid, but you don't need all of that to be productive with everyday front-end development work. Presumably anyone new and/or below senior level is going to be supervised/mentored while they're getting up to speed and have their code reviewed anyway.
That's how you know you picked the wrong framework.
I learned to use Svelte (granted, v1), in 45 minutes, and build something using it. I guarantee that a developer won't need 2 weeks to become comfortable in it.
"you don't always need to think about the ability to hire additional developers who know your framework of choice"
Unless you are hiring a junior dev who has no experience I think this statement is pretty far fetched. The ramp up time for a new dev on a framework that isn't widely adopted yet will be way higher, plus hiring developers who would even want to work with said framework sounds like a task in itself as well.
If you want to enhance HTML use Vue, if you want to focus on Javascript for everything use React. I like the idea of Svelte coming in to push other frameworks forward, but by no means do I think Svelte will be the future (this may not age well, but at this time my opinion).