A more convenient designer...

Support for .cshtml or any kind of new tech page like Blazor, Razor or whatever. Finally these are really html pages but wih some twists in the syntax which would not be very hard to ignore or at lease simulate them and let us design our page for Bootstrap integration which we do already manually. That would place BSS at the top of the chain ;)

Who doesn't use ASP.NET MVC, Core, Blazor and so on in Visual Studio these days ?

Right now we have to design Bootstrap, HTML and CSS just for layout in BSS and then transport all this in our ASP.NET MVC projet and modify accordinly Then maintenance in BSS afterward isn't possible and we are left with the old school which is modify our page manually. This brings us to why we're using BSS in these circumstances ?

This brings us to why we’re using BSS in these circumstances ?

Well you've pretty much answered your own question. Your requests (I suspect) are far beyond the scope of what the developers of BSS created the program for (or the capabilities of their intended audience.) Most BSS users are just building straightforward front end pages and uploading them to servers. I've built well over a hundred websites with BSS and other than a (rare) .PHP snippet to create a password protected page, I've never needed to execute server-scripts. I've certainly never brought generated BSS pages into VS or Atom, etc. For anything BSS can't handle, I go to Pinegrow.

Keep in mind, this is a $60 program.

I perfectly understand don't worry. BSS is lovely.