It should not be the default. This is almost certainly Lit's biggest flaw.
I am currently working on a web components framework, and I scrapped everything halfway through after I realized that you very rarely want that much encapsulation. Now, you can turn it on if you really need it, and I even made a way where you can pass references to DOM elements and stylesheets and such into each component so you can pierce the shadow veil easily. I have one demo to show it off, but I'm really having trouble imagining when someone would actually want to use it. The only time I can imagine it being useful would be building component libraries inside of large organizations.
I'm not ready to show off my framework yet, but I'm very certain that this leads to a better DX.