I was unable to come up with any suitable solution that also preserved PoS checkpointing functionality - IMO it just can't be done (the right way) with current hybrid PoW/PoS design. In PoS-only system - no problem. But with hybrid chain it's just one hell of a mess.
This should be weighed carefully. PoW is a bit hard to rely on long term due to not knowing how technology is going to affect it, not knowing if someone will plug in massive hashpower out of the blue someday, not knowing how future energy costs will factor in, etc.
It's not relying on PoW per se. In the future PoW can be disabled completely and the "no 2 consecutive PoS blocks" rule removed (block trust value would not matter then anyway, and the same goes for PoS checkpointing).
However, this is a no-go short term, as we're still pretty much in the initial coin distribution stage and there's too few PoS blocks to be able to rely on it (and remember we need a huge amount of different people participating in PoS, not just 2-3 big hoarders).
Also, remember the increasing computing and memory requirements of scrypt-chacha. If/when we see a massive break-through in hashing hardware, Bitcoin will be the first to go, not Yacoin.
If we happen to see scrypt-chacha ASICs, the difficulty will adjust MUCH faster than in Bitcoin ('cause we do a retarget after each single block, compared to Bitcoin's 2016). In case of huge energy costs and the network hashrate experiencing a massive drop - YAC can also adjust fairly quickly (just look at the Nfactor change events).
As far as quantum computing goes - we'll have much greater problems than cryptocurrencies going bust then.