The cartridge thing is a myth. It doesn't follow physics. Contact pins wear down through abrasion and normal derogation of PCB. PCB takes about 20-30 years before it starts to make a difference and it realistically takes 80ish years to see warping from age, but frequent abrasion from removing and inserting cartridges is an easy way to wear them out. Even then, my NES lasted 15 years of inserting cartridges and I got rid of mine before I could see the loader fail. Cartridges tend to go bad before the cartridge slot itself because once again, physics are in charge with the slot being stable while the downward force has more abrasion being done to the cartridge. You can replicate this with firmly holding an egg in one hand and smashing another egg into it. Majority of the time, the egg moving into the stable egg breaks.
The only downsides to keeping a cart in the slot are the possibilities of the console falling where the inserted cartridge is leveraged as a wedge to damage the slot as well as expose the PCB to air due to the dust/air cover no longer limiting exposure. Regardless, it does happen, but occurs so late in the hardware's use that it will have been recycled or thrown away by the time it is noticed. An example are the computers from the 1950s still being functional without replacing the parts. Everything should be broken if the advice was true. Modern hardware has more points of failure like spinning metal plates for a hard drive that will break down significantly faster than the motherboard it is attached to.
The Weebs fleeced my vidja collection.