It doesn't make any sense to me why this expansion is worse, not better, for 5 boxers.

PvE: same things as before except now you also get challenge modes. There are now more options, not less.

PvP: same as before with teams of four blowing peeps up allowing for a solo healer or rolling with 5. Same options as before the expansion, it's just that DKs may not be as good.

If the true question is can DKs do as much as they could in Cata, then maybe they can't. However, in general this expansion is better for five boxers, not worse.