Between an Asus ROG or a Lenovo Legion, which one would last the most in terms of years? Like, more resistant to heat and such, more quality?

I don't know

My wife is a heavy gamer. She upgrades to the latest ROG and Nvidia graphics cards every 2-3 years. There are three gamer adults in my household and all have been running ROG mobo since ROG II.

I have found the ROG very forgiving when swapping components and not using fully proper ESD procedures. If you are worried about heat - go with water cooling - and skip the cheap fans. Me, I don't mind if my computer sounds like a jet turbine engine firing up on the runway - lots of air is the way to go! If you need quiet - go with water cooled.

I have nothing but praise for the ROG series. Unfortunately, I have not used the Lenovo Legion.

Most computers are made largely from the same types of parts, and are therefore susceptible to the same issues, which is mostly overheating, shock from dropping or bumping, and power surge during lightning or extreme weather.
as long as you treat them well, even a cheap computer will last a long time.
case-in-point, i've had my current system for over a decade!
one thing that will make them last longer is NOT overclocking. Overclocking may make it run faster, but also wears out the parts faster, and not just the parts you overclocked!