The stat inflation is related to the 1.6 patch, with the implementation of the Champ system they also changed the way stats worked so that people could actually see the impact of champ points. So it's not a case of stat inflation in that gear has suddenly jumped to crazy levels, simply a direct conversion of old stats into new.
20k hp is the general aim for DPS classes. 15kdps is not the norm though. That is for extreme end with perfect gear etc. What you want to be aiming for is 8k dps - that is sufficient for all content in game. From there you can worry about perfecting your rotations and gear and tweaking your build to start pushing further.
Very, very few people reach 15k dps. A break down would be:
8k+ is entry level end game dps - enough to complete content and something your average guild would be very happy with.
10k+ is the advanced end game dps - this is where you want to aim to get to as you do more and more raids and beome more familiar with your build.
12k+ is what high end Guilds will be wanting their players to get, this kind of dps is beyond the capability of majority of the population.
15k+ is the high end, hardcore, top 10 on the Leaderboard chasers. A very, very small minority will be reaching this standard.
As a DK you can tank and DPS without issue. Even better, you want to be 2h DPS, which means stamina based, which means in terms of Champion system they work perfectly together - Stamina regen and reduced stamina costs is a big deal for both roles, so you have no issues making both tank and DPS
The Warrior constellation of the champ system offers nothing for DPS anyway, it is all about survivability which means focusing on improving tank without sacrificing your DPS. The Thief is where the Stamina regen and reduction I mentioned before is, and is pretty much all about resource management, so focusing on those two is most important for your roles. Mage offers nothing for tank but The Ritual is dedicated to stamina DPS, so your Mage points would go all in there, again, having no negative impact on your tank.