I think it also depends on how the template is built on that site. Some would be more difficult than others. I have successfuly bought "Landkit" from that site and have the whole shebang converted, sass, images, js and also the html converted too.
I learned quite a bit.
I would say if I repeated the process I would "not" convert the html but leave it as custom code. Then since Bootstrap Studio now has the ability to open html, css, or js files in a external editior I would start creating "snippets" or "components" from the html opened in the editor itself and save them there. In some respects this keeps from cluttering up my BS components from other external theme components. Since the components are well designed and structured and use the Bootstrap classes for design, you really are just ending up changing the pictures, links, and text. Colors and fonts are handled globally with sass so again its really not much of a jump the user is taking from any other editor that you would be using, and you get all the extra benefits of Bootstrap Studio which may be missing from other sub par editors or over-complicated hard to learn editors.
I also wonder why some of the webmasters on here don't look at this as a opportunity to make additional income from someone who is trying to keep control over ones own website. Offer your service to convert themes into Bootstrap Studio ? Sell a customer on the idea that they can pick a theme, purchase Bootstrap Studio, and you will "hand it off" to them after the design is done ?