I quite liked svelte 4. Svelte 5 might be even better, but I'll probably never find out.
I won't touch it if I can help it. No matter how good it is, I'm fairly certain there will be a svelte 6, 5 will be abandoned, and whatever issues crop up in it will not be addressed unless I move to 6... probably just in time for 7 to come out. I know this because that's what happened before, repeatedly.
I don't know if htmx is actually any good, but they are saying all the right things.
Major versions that change basically everything should be considered different projects and require a re-review. I remember I did this early with react-router, and I think when they changed the public API for the second time, we forked it and left it as-is, because they made their goals apparent.
If Svelte version 4 and 5 are very different, then they're "SvelteV4" and "SvelteV5", both on version 1.0.0, in my mind :)