"Due to many requests, the submission deadline has been extended" is this real?
Warning: personal opinion.

I view conferences that regularly shift submission deadlines as a little suspect. You're right that this happens in CS conferences, and it soon becomes pointless because everyone expects the extension ("Nobody expects the extension!!"). But your estimate of 80% is rather off. Almost none of the (many) conferences I submit to have deadline extensions for submission of the full paper. 

And don't underestimate the level of pleading that goes on. People will always ask for an extension if they have even a smidgen of hope that the pleading will work. It's only when a conference takes a firm stand for many years that people stop asking.