>The issue with PoS, of course, is that while the work function is insular, purchasing stake is not.
That's not the issue, that's the 'S' in PoS. Consensus partially breaks if majority of state is hostile (no imminent double spend risk unlike PoW, but may require a manually coordinated fork with minority of validators). It breaks for 2/3 of hostile validators in the sense that new nodes would follow a different chain while honest minority is building on its own fork.
Both failure modes require some manual coordination, but rather than successfully censor or steal, they create temporary chaos - at the cost of millions of eth. It's impossible to even buy enough now, and any attack is self extinguishing because the available supply shrinks.
Under PoW the failure mode is trivial and deadly. If the attacker owns majority of hashrate he can censor anyone forever, including other miners. There's no real defense because there's no in-protocol way to delete the attacker's asics. Switching to a gpu pow achieves nothing - because the only entity prepared for that would be the attacker itself - but it would be pointless in the more general sense, as nobody would ever trust PoW anyway, because the same attack can always be repeated. The issue is use of external resources not controlled by the protocol.
Attack from the state is one thing, infinite economies of scale is another. An entity controlling majority can multiply its income by censoring everyone else. Eventually, everyone else mining at loss disappears. Then they can only mine at a fraction of hashrate and instead of paying for energy, use a fraction of excess profits to buy more asics. In the case of any competitor they would turn all of them on again until competitor gives up. This one is eventually inevitable, hard to say how soon.
These issues are completely unsolvable in PoW.
>so Infura type entities are not necessary
It's just a rpc - it's always necessary to answer queries and send transactions. At best it could work inside js in metamask as a light node. The only theoretical attack infura could do is lie about balances to someone, but the discrepancy would be detected and that would be the end of people trusting infura (to report correct state).