Okay, let's play that game. Tractor Data says this about the 111:
Original price was $2,279 in 1983
Inflation says the equivalent price today would be almost $7K
The 2.80% inflation rate means $1 in 1983 is equivalent to $3.02 today. This inflation calculator uses the official US consumer price index published by the Department of Labor.
The X570 list price is $7,800 today, so about 12% more than the adjusted price of the 111 from 1983.
In 1983, you got a 11HP engine, 38" deck, 5 speed gear drive transaxle and manual steering. In 2023 you get more powerful twin cylinder Kawasaki, 48" deck, ground engaging transaxle, but still manual steering (you can get power steering for only a little more).
Now, if you compare the 111 from the 80's to the S100 series of today they have similar prices in "dollars" but a dollar today is worth a LOT less than it was in the 80's. I am pretty sure there will still be lots of X500 series machines around 30+ years from when they were made, but probably not so many of the (letter)100 series machines. You are correct that most of them have a design life of a decade or so. While you might be willing to pay what it costs to buy something that really lasts, most people are not. They focus on the initial cost and don't think of the total lifecycle cost and also don't really want to have to take care of something. For them, the disposable lawn mower is exactly what they need/want. Deere makes those and they are very popular. Just don't confuse them with the products that are made for a different target.