But, if I purchase the miner instead and the price drops to $70, I always retain the option to sell the miner. For example, selling the miner purchased originally at $1500 for $1000 at a $70 conversion rate, I can receive 14.28 btc for it, and that's on top of the few btc I would've mined already.
Yes you do have the option to sell and earn like you have described.
Notice that if you do sell if for 1000$ or 14.28BTC, together with the BTC minded before selling, you could end up with more than 15BTC!
15BTC which were impossible to mine directly with that miner! (according to your scenario)
but the only question is, who will buy it for 1000$ in this situation? (assuming no stupid people in the world)
Why buy a miner for 14.28 BTC with no chance to mine 14.28 BTC with it? ( "No chance" becuase this is the situation you have described).
It's is better to just buy 14.28 BTC directly rather than buying your miner!
However, the value of an ASIC is not "purely" determined by the BTC it generates because the value of an ASIC in terms if btc is mediated by fiat conversion rates. If I spend $1000 on an ASIC that makes 0.1 btc per month, it's a good decision if the value of btc is $10,000. It's better to receive 0.1 btc valued at $1000 than to receive 1 btc valued at $100.
Here, again you assume that one will sell you a miner for 1000$ which will make you 10000$ a month (0.1BTC).
Who is stupid enough to do that?
You can play all day long with hypothetical scenarios which requires a stupid buyer/seller but in the end it all comes down to this :
To buy and to mine are both equal to acquiring BTC.
The better way from these 2 options is the one which ends with more BTC in your wallet than the other.
And in that same "better way" you would also have more USD comparing to the other way (becuase more btc is more USD).