tl;dr: Yes, there is DR, but its effect is too small to make a difference for players that hoard them.
You're absolutely right, there is a diminishing return mechanic on the mainstat. But actually, Paragon points are kinda infinite, so even though they have lesser and lesser effect the more you have of your mainstat, if you have like 50000 mainstat, despite DR you're dealing insane dmg. there are players out there with over 10000 Paragon. It's crazy.
Sadly, the meta is to get as many Paragon as you possibly can and then push rifts before season end