Can't we just slow down PoW block target rate, so only a couple are mined between PoS blocks.
That wouldn't stop anyone from orphaning all the PoW blocks at the tip of the chain.Yeah, but orphaning a single PoS block with 2 PoW blocks actually costs you something (you have to be lucky or have majority of the network's hashrate).
It seems that there's currently a pool in a position to make it happen on a frequent ongoing basis. Orphaning a PoS block with 2 PoW blocks doesn't really cost you anything extra if you're already PoW mining anyway and happen to hit 2 consecutive PoW blocks.
Nope, the rogue PoW miner has to decide in advance to invest his work in what will most probably be an invalid chain. So when he fails to execute his attack, he effectively lost his potential profit from playing nice.
OTOH, with PoS you can safely and with absolutely no additional cost mine on all the forks.
OTOH you can generate PoS blocks essentially for free.
Well, you do have to hold YAC, and hold it long enough, so I'm not sure it's correct to say minting PoS blocks is free.
(and trust me, the amount of YAC is small enough for anyone to buy)
Also, the gain from orphaning PoS is ZERO.
The gain in the miner's held YAC from orphaning PoS blocks is zero, but there may be other motivations other than an immediate gain in YAC to orphan PoS blocks.