In the past, people farmed and exchanged the surplus grain harvested in autumn for rare, unchanging gold. Then give the gold to a trusted person in exchange for his IOU.
Not really!
For most of ancient history and medieval times, farmers either exchanged their goods in barter deals, this extended till the 19 century for some parts of Europe or they would get paid mostly in silver. One as to look at what the roman empire run on and it wasn't really gold, more like silver with bronze and copper battling for number two spot. As for the IOU, that never happened for the average farmer or citizen or anything, the first form of IOU was solely used by merchants and was totally inaccessible to the masses who could barely read and write their name.
The whole thing of gold coins being used when paying for everything in a medieval town is just Hollywood, in reality, it didn't happen. You would see a hundred times more people splitting silver coins into two or four pieces to pay for stuff than owning a gold coin.
Now and in the future, we will replace the surplus production with a rare, trustworthy and unchangeable Bitcoin, and it can be quickly exchanged for the product we want in any corner of the world. We no longer have to work hard to carry these gold and IOUs.
You could do that for decades prior to this..
Your oversimplifying things, it's not like you have excess production and in a matter of seconds you can sell it for bitcoins, trust me! I've spent way too much of my life dealing with the farming business and you have no idea what a pain in the ass negotiations over a few cents per kilo is, the speed of payments is your last concerns, securing cheap resources and getting a good price on it are for more troublesome than actual work. Bitcoin is money, if you can't sell your product for money then you won't be able to sell it for bitcoin either and the chances are far worse with the second.
Be honest about it, when was the last time you purchased wheat or meat or eggs with
BTC ?
That being said, I don't know why you made the title about productivity.
Productivity has nothing to do with bitcoin, bitcoin excels at transferring wealth or storing it, it won't increase productivity simply because you get paid in it.
It might be like this in the future but at this point, the major path is still goods > fiat >btc > fiat >goods.