That said, I think both are pretty unlikely, assuming no big changes to the code. Very hard to secure the network at that pricepoint, without building many many power plants...
Have you actually done the math on that? Based on hash rate trends, where would you expect the hash rate to be in the case of million dollar BTC?
The key here is not hashrate precisely - that depends on technological advances.
Rather, I had in mind marginal costs - electricity - because the difficulty adjustment is in essence an economic peg: the average miner's marginal cost to mine approaches the block reward.
Today, whether clean or dirty energy, the marginal cost to generate electricity is 5-10¢/kWh.
Thus, a hypothetical world of 2021 with 6.25 BTC block rewards, a $1/microBTC pricepoint, and power plant costs on the low end of that scale (if we assume tech there continues to improve), would be using an extra ~750GW of electricity. If we are talking solar farms, then it's solar farms that add up to about the acreage of Switzerland.
Not impossible, but still dizzying to imagine as a physical implication of a price change.