well my 2 cents, to be honest at the 4-5,000 mile mark, most 2 stroke motors are due to being checked for compression as that is honestly getting towards the end of there top end life span before things start to go south, its NORMAL WEAR AND TEAR, nothing new here(older sleds adn motors were even sooner to be honest so new motors are getting better)
NOW< at 4-5,000 miles it doesn't MEAN they NEED a top end, it means things should be checked and then checked more often as more time and miles add up.
there are countless folks that get more than twice this 4-5000 miles (many get 10-12,000+ miles on modern 2 stroke motors) and then some on miles on modern 2 strokes problem free, but then there are also many that DON"T!.
Sleds are wear and tear toys, like it or not, HOW there driven, where there riven and how there cared for in seasons and off matters as t how long many things past, you cannot just look at a used sled and say, LOW miles GREAT High miles RUN!, its not that simple, every other wear part on a sled has to be taken care of they all work together to either make for a well cared for sled or not
4 stroke motors just have longer intervals before top ends are needed, there not magically more reliable by any means, there just a different design and as such they have different intervals on things! rest of sled is the same to wear and tear!
there are LOYAL followers of every brand that have GREAT luck and long life of there BRAND sled, ain't no way to say buy this make or model in a used sled, as how it was cared for or not matters more IMO, than just picking a yr or model ever will!