I’m not sure you are talking about blocking workflow or about “modeling” 3D assets in engine. If it’s about “modeling” then that’s true. If it’s about blocking, then you are wrong. Blocking out level is not a historic way (maybe you don’t know what blocking means?). To recommend Sketchup for prototyping (blocking?) sounds confusing. You don’t see the advantage of blocking out stuff right in the environment where the game will happen? What about level that’s work in progress and you want to do some additional blocking? Would you be exporting the already made parts to sketchup and then continue there?
I think blocking is a valid workflow, UE doesn’t have bad tools for it (even more so that they have been updated), but still they could be enhanced. Only one of the problems with BSP - long or longer undo under different circumstances, that can kill a smooth workflow. But if OP really means “modeling” of the whole level from start to finish in UE, that would be too much for these days. For AAA engine like UE it would require integrating almost maya/ level of 3D modeling tools (which of course is out of question).