I think you may achieve shorter URLs with a special add-on we have, "Keyword-Rich URLs
It will not critically affect the file size. All such URLs are specified just in few places, and later they are build dynamically.
Also, it's not really secure to use multiple files instead of one. Now security routine passes through the index.php and it then includes the proper module. In your suggestion, routine should pass through many modules, and if it's updated, it may be also updated in many modules. It's not productive in terms of development.
Thanks for suggestion, but after running miniBB for years with the present URLs, I think, I will not consider it :)