Excellent question, major misconception spread on YouTube view hit collectors that it is a 500,000 truck and when you buy a 250,000 you have half life left.
According to various iseecars statistics and each year slightly different, there are 18.2 % land cruiser past 200,000 miles (or roughly 2 out of 10) and and 1.5% past 300,000 (or roughly 1.5 out of 100). According to that statistic alone, there is a giant cliff drop between 200,000 and 300,000 miles, your mechanic may be correct.
It also holds the first spot for cars on the road with most miles at both 200k and 300k, because of relative reliability and high cost of the car that it is worth a lot of repairs, but... Once it reaches higher mileage, drop in equity cost in the car vs costly repairs make it not worth it, just like any other car. They are great but not indestructible.