In Duels you can get away with it by just deleting cards that aren't helping. It's a benefit of the online format.
Yes and no. They've already done card substitutions for power level reasons, but those were all on cards in the starter pack (the ones everyone gets by default). When you start doing substitutions for cards that come in booster packs (that people may have paid for) it's a bit of a slippery slope. They risk upsetting their player base. They've done one substitution already, but it was because the card was buggy and too much work to fix; not for power-level reasons.
The critical mass issue shows its head the most on the mana base. For a long time, 4 and 5 color goodstuff decks were the best way to go because the format is a lot slower and fixing was plentiful. They've since substituted some of the problem cards, but their hands are now tied on adding new duals to the game. The new fast lands were skipped and I bet whatever duels show up in Amonkhet will be skipped, too. They did, surprisingly, add Aether Hub, which was probably a mistake in the long term, but we'll see.
Of course, there's also the chance that they already have a time table in place for Digital Next. If they know that Duels will be phased out, it could be that it's soon enough that they're not overly worried about the long-term implications of the card pool.
