While I personally don't mind the idea of a dual system, I think you do have to consider that the gears rewards for dungeons are most likely considered an appropriate step forward in gear progression for the minimal ilevel and content type. Consider the following:

3.4 Dungeons
Min ilvl = 210
ilvl of Gear = 225
+15 ilvl from min

3.3 Dungeons
Min ilvl = 200
ilvl of gear = 215
+15 ilvl from min

3.2 Dungeons
Min ilvl = 180
ilvl of gear = 195
+15 ilvl from min

3.3 Casual Raid
Min ilvl = 205
ilvl of gear = 230
+25 ilvl from min

I've omitted 3.0 and 3.1 because to me it's clear they wanted to (unsuccessfully) shake up the ilvl arrangement via how they did weapons and raid drops and reverted back to a more normal system afterwards.

If we follow your request and using the same trend S-E seems to want to keep, current dungeons would have to have a minimal ilvl of 245 and drop i260 gear (Scrip gear), effectively locking out anyone in the same way it was pointed out about how 3.0 "locked people out".

Of course that means they could change the system to allow for higher ilvl gear to drop but I imagine S-E would not like that either. Remember, even though 90% of us are already decked out to the brim from the previous patch cycle to get into any content doesn't mean the new players are. The dungeon gear might be useless for most of us but is still upgrades for those who are new or perhaps gearing alts and S-E deems these dungeons to be an appropriate power creep for the purpose it serves.