I am writing about what you said:

> The file won't be useable anymore by the Windows explorer (bad extension)

So what if they can't be loaded just by clicking on them?  What if a user,
such as myself, wants to see the downloaded website with (almost) the same
file names?  

But also, many web downloaders such as wget and WebCopier Pro has an option to
add the HTML extention AFTER the file name and converted query string:

page.html@param=value&param2=value2.html

I believe Natan is right.. it would be easy to do.  When I look at my file
directlry and only see index####.html what does that mean to me?  I know the
file name by heart on the live website, but how am I to quickly find and edit
it on my computer???
-Rich