Words are worth debating.

I fully agree. It's also worth noting that a number of 'kinder' insults we use today are rooted in exactly the same definitions. According to Merriam-Webster:

Idiot, imbecile, and moron were, not so long ago, used in a psychological classification system, and each one was assigned to a fairly specific range of abilities.

Idiots. —Those so defective that the mental development never exceeds that or a normal child of about two years. Imbeciles. —Those whose development is higher than that of an idiot, but whose intelligence does not exceed that of a normal child of about seven years. Morons. —Those whose mental development is above that of an imbecile, but does not exceed that of a normal child of about twelve years. — Edmund Burke Huey, Backward and Feeble-Minded Children, 1912

To that end, I assume "Retarded", which literally means "delayed" came into common parlance to replace the above definitions with a kinder word - which it would have been when it first came into use. However, it too became a slur because it was used to compare non-delayed people doing dumb things to people who did not have the mental capacity to realize that certain actions didn't adhere to the common norm.

So the root problem isn't the word; it's the comparison of the actions of those who should know better to those who lack that facility. This is why we've now switched to 'developmentally delayed', which is another, longer way of saying what 'retarded' means. In another 20-30 years, 'developmentally delayed' will also be a slur.

So, the whole cycle is all pretty retarded if you ask me.