What you fail to grasp is that yes, we know the S9 will eventually become worthless. HOWEVER, that is only if we decide to hold them long enough that they are impossible to sell.
On the contrary sir, I understand perfectly. That's what started this whole thing, just look at my first posts in this thread.
As I mentioned many times before in this thread, I was mining with my beloved S9 till just over a month ago.
I saw they were selling on Ebay for ridicules prices and did some calculations to see if my miner would still make me as much money as the profit I was able to sell it for at that time. I posted the resulting spreadsheet here, the outcome was obvious, SELL!
So after mining for a while and making some descent BTC, I ended up selling my 6 month old miner for $2600 more than I originally bought it. I'm now investing that profit directly into crypto (not BTC at this time but that's another discussion).
Anyway, since then, miners on Ebay have halved in price. I sold at the peak it seems.
So the question for you is, do you sell your miner(s) today and probably be able to get your full original investment back (that's what they're selling for roughly now on Ebay) on top of the BTC you earned so far, or,
do you wait a few months and sell it/them for perhaps a couple of thousands less? And how much BTC will your miner mine in the meantime to make up for that difference.
It's a hard decision in any case, mining is just fun, I know! Yet, rationality should prevail.