Makes me wonder how fancy and expensive those walk behinds are. Nearly every low-end walk behind mower I've used never had engine problems despite zero engine maintenance. They only got replaced due to the handles or wheels breaking, the chassis rusting, etc. I suppose they may have gotten a new spark plug once every many years.
I helped go through someone's belongings when they died. I was quite surprised to find receipts for a yearly tune up on their walk behind. They paid a shop to pick it up, bring it to their shop, replace the spark plug and air filter, then bring it back. This was done every year on a mediocre lawn mower used on a small 30 x 75 or so feet lawn.