You could, but it wouldn’t achieve anything new or correct. Your items will still be of a higher quality than they should be.
Instead you look at Main Stat. Since every GR tier adds 17% HP to monsters, the requires Damage to kill those monsters is 17% higher. When All is maxed out, the only way of gaining 17% more Damage is to increase Main Stat 17%. So instead 20,000 Main Stat you need 23,400 Main Stat to complete one tier higher. Now this is dependent on Gem ranks aswell, so back when 100 and 120 was top the Main Stat per Paragon Point was lower, as 1300-1560 x 5 Main Stat from those was the max to expect. Now adays GR 140+ is more common so 1850 x 5 Main Stat is more likely from that, and by the time you reach Paragon Level 5000 you most likely have those 150 gems.
So working from that it is easier to evaluate down in terms of Main Stat and see how something performs. Compared to upscaling from under-optimized gear and and gem ranks. That is also why you see so many push builds in PTR testings report their current Main Stat. And a lot do actually test around the 10K mark because it is simply easier to scale off.
This is what Blizzard does, they increase Main Stat to an equal point which they have set to paragon 5000, as that would be the point where you are sure to be optimized. Secondly at 5000 you are more or less going to double your Main Stat when you reach Paragon Level 10,000, effectively meaning +5 GRs or potential GR145. So just below the cap.