>> This is interesting. I am curious, which PoS coin implementations do you like?
I've been involved with Lisk (LSK) for several years. It's Delegated Proof of Stake though so you can use your LSK to vote for block forgers who offer a good % share of their block rewards and earn interest that way.
>> If an attacker wanted to not raise the price of a coin, couldn't they use crypto OTC markets to buy large amounts while not raising the price
Yes, that can happen in theory but in practice it's not feasible. In DPoS especially, whales would rarely agree to sell more than 50% of their own stake because if they did they could lose their forging delegate spot (which yields higher rewards than just voting). Because the blockchain is public, delegates all watch each other's on-chain activity and they can lose votes if they try to sell too many tokens (doesn't matter if it's OTC or exchange).
>> Aren't cryptocurrencies also only a scarce resource if they have a hard supply cap? Or do you figure in something like inflation vs coin burning to this as well?
If there is inflation, it doesn't affect the security of the blockchain because the attacker must acquire 50% of all tokens in any case. The more tokens there are in total, the more tokens the attacker needs to buy to get to 50%.
>> The only reason I could see the attack making financial sense at that point would be if it was a competitor who was trying to kill a competing PoS network and it was worth it to them to do so in order to promote their own network
Early days of any blockchain are always more risky. That said, the early days of most PoW blockchains are even more precarious than those of PoS. This is because with PoW, the community has no say over who can start forging blocks on its new blockchain (anyone who owns some crypto mining hardware can compete to produce blocks on potentially any PoW blockchain).
If just a tiny % of Bitcoin's miners were temporarily repurposed (e.g. minor software changes) to mine any new PoW blockchain, those miners could easily take over the new blockchain and create any transaction they want. With PoS, in the early days, the community gets to decide who will receive the initial tokens; so outsiders cannot highjack the network unless then find a way to buy more than 50% of the tokens from existing token holders.