Does a waterproof mattress protector really help with longevity?

Absolutely!!

By preventing moisture, body oils, and dust mites from getting into the upholstery layers of your mattress, you keep those layers from breaking down prematurely.

Also, by keeping your mattress clean and dry, you can prevent the upholstery layers from absorbing your body odors, which keeps your mattress fresh smelling.

And, any kind of stain will void the warranty on your mattress, with perspiration stains being the most common. A waterproof mattress protector prevents that.

Organic Cotton Waterproof Mattress Protectors, made by Sleep & Beyond, are what The Mattress Expert uses at home.

Sleep & Beyond Organic Cotton Waterproof Mattress Protectors/Encasements

https://www.themattressexpert.com/sleep_and_beyond_protectors_and_encasements.html#/