A lot, if not all, of the big farms started when btc price was less than $500/BTC; I think they will remain profitable for some time to come...
btc value is not the issue at hand - profitability is.
You state they were profitable when BTC was under $500 that was also when BTC reward was 50 BTC not 25 BTC as BTC was at around $650 when the 2nd halving occurred. Though it was at around $300 about a year before the 2nd halving - if I recall correctly.
What I mean is revenue, which will control the profit. However, there is more to revenue than just the BTC value. There is the transaction fees - which could go up and or down, there is the difficulty when it dips the amount of BTC per TH earnings increases and if the difficulty increases the amount of BTC per TH decreases.
but as with the prior halvings it has shown that with the decreased issuance of BTC has caused the value of BTC to increase to match the supply/demand of the market.
but if the BTC value does not increase accordingly than there has to be some way for the farms to keep the revenue up. One of the things working for that is the increased efficiency of the newer miners. For years the efficiency was around 80J/TH but now with miners getting around 40J/TH - This increase in efficiency will help boost revenue by reducing the cost.
There are many factors indeed and farms will work to remain in the green, of that I agree. But if BTC value goes down instead of going up - it will still tax even the best of farms.
I should have went more in depth with my thoughts -and I acknowledge that this is my opinion and could be entirely wrong - but as with all speculation, it is just that speculation.