Why Descartes should have stuck to math.

I doubt you will find any argument from any philosopher, scientist, or mathematician that Descartes wasn't brilliant.

With that said, I would like to show why he was merely a subpar philosopher.

The pineal gland is an area of the brain that really wasn't understood until recently, but just because you don't know its purpose doesn't mean you can simply throw your metaphysical theories into it. In "Meditations On First Philosophy" Descartes refers to the pineal gland as the "seat of the soul." Descartes entire philosophy was centered around the soul and the body; however, he was never able to explain how the two coincided.

Out of desperation he looked to the pineal gland to solve his paradox. Its function was unknown at the time, so he decided that it was the perfect spot for the soul and body to interact and "do their thing." Even in this time period most people told him that this hypothesis was nothing more than crap.

This is exactly why philosophy depends on science and not the other way around. Just like how any philosophy about the origin of man, the morality of man, or the purpose of man written before 1859 CE should not be taken seriously and only read to understand the philosophy of an unfortunate, pre-Darwinian era.