Do waterproof mousepads actually stay waterproof forever?

No. The waterproof coating (usually polyurethane) repels sweat and oils for months, but your hand’s natural oils and friction break it down over time. Cheap spray-on coatings wear off in weeks, while bonded coatings on $20-30 pads last six months to a year. If you sweat a lot, a hard-surface pad is a better long-term choice.