I'm rewriting a bunch of my Vue components into Svelte, and turning my Nuxt (which were previously React/Next) projects into Sapper, and it's a crazy easy process. My Svelte codebase is tiny compared to my Vue codebase.
I'm slightly "worried" about Sapper not being at v1.0 yet but... it's been working great for my MVPs
TL;DW - parts of Sapper may be in a future version of Svelte, but looking to add support for mixed static and dynamic pages and improving development speed to change by using Snowpack.
I'm pretty out of the loop when it comes to Svelte/Sapper. I've looked at it, but never used it. However I did recently spot that they're already working on a successor to Sapper called Svelte Kit. So maybe that's why Sapper isn't at 1.0, and maybe never will be.
I'm slightly "worried" about Sapper not being at v1.0 yet but... it's been working great for my MVPs