I agree with Mark (e57) RNC (Rigid Non Metallic Conduit) even schedule 80, does not come close the the strength of RMC (Rigid Metallic Conduit)
That said we use plastic almost exclusively underground for the same reason you brought up. Metal conduits rust out and even when they do not rust through they rust enough to make the conductors stick to the conduit so replacement can be tough.
I do not believe you will see the burial depths for RNC reduced. Take a look at Table 300.5
The burial depths for RNC range from 0' under a building to 24" under a parking lot.
It should be legal to use at a 6" depth like metal rigid,
That depends on the location, the burial depths for RMC range again from 0" under a building to 24" under a parking lot or road.
As a matter of fact there are only two places in the table where the burial depth differs between RNC and RMC.
1)In trench below 50-mm (2-in.) thick concrete or equivalent
2)The row titled "All locations not specified below"
Keep in mind very few locations are "not specified below"
in the table.
You really need to study the table, if you are in an area with tough digging you can choose to run the RNC at 12" with 2" of concrete on top of it.
We do mostly commercial jobs and many times the specifications call for RNC at 24" depth with concrete encasement.
Of course a machine is doing the digging.