2p/4p will probably be a factor during progress through heroic, but I think it's likely to substantially go away by the time guilds are progressing through mythic just because the raw loss of stats will start to be too large. Even if you're lucky enough to have two 920+ T19 pieces you'll need to have the right Tomb pieces at high ilvl/good itemization also, and it just doesn't seem likely to me that very many people will be in that situation for long.

plus, some targeted nerfs to 2p bonuses have already been made and I expect there'll probably be more if the T19 two piece bonuses are a big draw (especially for stuff like the resto 2p that would just need a numbers reduction.)

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also a lot of the reason that the drape of shame is so good relatively to ilvl is that cloaks have pretty low stat budgets; you don't gain much int/rating as your cloak goes up in ilvl compared to other pieces. If the drape were (say) a chest, the incentive to keep it over a higher-ilvl piece would be much weaker.