My 10 year comment was meant to combat the dumping issue and 3 years for a $4000 purchase so in your mind 1300 a year is a fair amount. I think you know engines and wish you lived closer but the rest of your comments make you sound like an employee so no more needs saying.
First you said 4 years not 3 years. Second I own my repair shop so yes I am both the owner and employee. I know what replacement parts costs are and it is different then the cost of complete units. I have at times brought new walk behind mowers just for the engines and then sell off the other parts later.
So you think the $4000 for a whole mower is just the cost of the engine. There is a lot more to the cost then just an engine involved. Below is my distributor FR691V list price on just one version of the FR691V.
So over half of the $4000 is the rest of the mower. Just wait until you find out what it costs for just one hydrostatic transaxles. They are around $800 list each. Then you throw in the deck housing at around $600 (spindles and other part are extra) and the seat at around $425. And don't even get you complete assembled mower. So the overall price of the mower is a considerable good deal. It is because the OEM gets a huge discount for volume of purchases that they can sell the complete mower for $4000 and make any money on it.
On top of that try pricing what it would cost you to have someone to mow your lawn during the same time frame. There are customers of mine that had to pay $150 each time they got their 3/4 acres mowed while their mower is in the shop awaiting back ordered parts.
I just repaired a JD Z925A last year that sold for over $10,000 and it was $2500 just to replace one transaxle and it took me 8 hours to replace it as it is a royal pain to pull the whole rear assembly just to get to the transaxle mounting bolts. And there have been a lot other repairs done on the mower over the last 4 yrs too. Then his Z950R needed a $600 carburetor just this week. The customer has actually spent about $6000 over the last 5 years maintaining the Z925A mower and he about due to replace the motor as it using a lot oil now.