A slightly warped desk won't ruin your extended mousepad.

Probably not. Unless the warp is dramatic enough to make the mousepad sit like a ramp, most modern optical and laser sensors handle a tiny bit of unevenness just fine. If your desk has a visible dip or bump, a thick cloth pad will conform to it. Even a thin hard pad won’t cause pointer issues unless the warp lifts a corner off the surface entirely. The bigger risk is the pad sliding around on an uneven desk, but that’s a grip issue, not a performance one.

The only exception: if you’re using an old ball mouse or a very cheap sensor from ten years ago. Those require a perfectly flat surface. But anything you bought in the past five years will track through a millimeter or two of warp without you noticing.

Don’t buy a new desk just for this.