It adds complexity but then if you need caching, what doesn't?
It sounds like I'd still get a poor UX on the first page load (UI checkbox jitter), and on all page transitions I'd need to run the user specific lesson completions query produced from the ajax request and all of the other heavy content under the video (questions + show notes, etc.) would need to get rendered which in the end is much more expensive than using frames to avoid any of that from being processed.
I'm not trying to make the case of sticking with frames + streams either, I think it's been kind of painful to use them with all that's going on within the page. I'd love to be able to use the morph approach. I think I'd be able to delete tons of both server side and client side code.
<div data-turbo-permanent>...</div>
I think it's not yet documented since turbo has not yet landed in a production release of Turbo but it's been merged into main, you can see it mentioned on the PR: https://github.com/hotwired/turbo/pull/1019 (Look for "Exclude sections from morphing" title in the PR description).