It sure does, however, there is a downside to it which is the cost per hash, an S9 will most likely last a lot longer running underclocked at 9.5th instead of the default 13 to 14.5th, but then assuming the miner costs $400 shipped, it becomes 28.5$ per th vs 42$ per th, you do save on power but the most often the lost hash rate surpasses all that.
Buying different batches might be a good idea, but not in the way you mentioned, batches come out in quantities of thousands, usually monthly, so buying from different people does not mean you will get a different batch, they could all have bought that one bad batch you were trying to avoid.
Usually, the first a few batches are the worst, when a few months have passed and the manufacturer starts getting a ton of warranty repair tickets, they "might" fix what seems to be the issue, but then missing the first batch might also mean paying a lot more for the same miner, it's pretty hard to time this, luck is a major factor.
It's both, for an example antmine S9 are known to have a dead middle board due to a lack of airflow around the first a few chips facing the front side caused by the fan motor itself, so the longer it runs the likely to fail, so this gives you a linear curve more or less.
With something like 17 series where very often the issue is just one bad solder ball shorting the shit out of the board, it could be safer to buy a used miner which has been repaired, if your 17 series miner makes it through the first 6 months, it's more likely to survive another 6 months, but then again, different batches will have different results, the ones that later came from Malaysia were a lot better, it seems like the robots/people that did the soldering were doing it in a better way.
I also remember buying an early batch of S9k where I got miners without heatsinks attached, the heatsinks were sitting in the case doing nothing, it seems like they forgot to heat the thermal , on the other hand, I recall Phill got a different batch that arrived in good shape and worked for a while (not sure about the current status).
So ya, it's just a mess, very hard to predict and it's part of the risk you have to take when you are looking at a business than can get you a 1-2 year ROI, I highly doubt that even drug dealers have such a short projected ROI, , this is why when you invest in mining, you can win big just in the same way you can lose big, it's a very risky business, especially for those who are just starting, I always tell the majority of people NOT to invest in mining, at least not before knowing the full story, because things look pretty damn good on paper, you go to an online calculator and think to yourself that you will get your money back in 6 months, reinvest and double your hashrate in 3 months, get that many PH worths of hashrate and start making 10k a month while doing nothing, it's exactly how things look like in an excel sheet like the one you made, but that couldn't be further from the truth.
I grabbed 4 s9k. they still all work. but two of the four would over hear. simple solution was detach the center board. they are working as i type. since sept 2018.
I dca gear. ie 4000-7000 a month in buys every month.
it usually means 1 or 2 pieces a month.
depending if it is asic or gpu it could be 1 or 2 or 3 pieces a month.