We all respect looks like your memory is tricking you ;-)

The reason behind sudden payout drop was probably bitcoin reward halving which is taking place every 4 years.
But the dates you are giving must be wrong. Halving took place 9th of July 2016.

https://www.coindesk.com/live-blog-bitcoin-halving/


Moreover difficulty is adjusted every two weeks:

https://bitcoinwisdom.com/bitcoin/difficulty

Right now upkeep fees on open ended conract not lifetime as you said are ~25% lower then 18 months ago.
Also it is not 20 days but 30 continuous days to stop your contract if upkeep is higher then your mining earnings.
This is crucial because mining diffulty take place every two weeks so in 30 days it is twice in 20 it was just once.

Why this is important? If price drops significantly less people mine and it is taking longer to solve the blocks which are generated on avarage every 10 minutes.
More time let BTC code adjust and lower difficulty to the level on which mining become profitable again.

Please see below helpful sites to understand the trend:
https://blockchain.info/pl/charts/hash-rate
https://coin.dance/nodes

As you see nowdays BTC mining i far more sustainable and is backed up by huge investments into mining farms.

Do you think it is coincidence that contracts are sold out and more and more huge mining farms are growing around the world?

Kind regards, ptaq