Those old engines were not made with anything in mind but longevity. Lots of 50 year old Wheel Horses etc still out there chugging away!
Back then the market was prepared to pay a reasonable amount for a quality item so there was reasonable profit in making mowers of good quality.
But people are greedy and always want more for less, profit margins got squeezed and thus the quality dropped like a stone.
On top of that people can not think about prices, all they remember are numbers so if a mower was $ 1000 in 1980 they expect better in 2018 for the same $ 1000 which when adjusted for inflation is about $ 400..
The idea of converting to real cost ( hours of your labour ) is apparently way too difficult for most to understand.
Back in the 60's 70's & 80's you had to take out finance to buy a ride on mower, now days you just pop it on the card.
In fact in the 60's we had to take out finance to buy our push mower and pay it off over 3 years.
We still have that mower and the only major work done to it was to convert the blade disc to take modern blades.
My sister who is over 70 still uses it to mow the common area around her home unit ( flat or condo to some )