The sheet mask no one needs
On the product category I gave up two years ago — and the simple wet-cloth substitute that does the same job for about a thousandth of the cost.
Sheet masks were the small wellness purchase I made most often in my thirties. The package said twenty minutes, the experience took ten, and the face, afterwards, felt slightly more hydrated than it had before. I would buy them in packs of ten, use one every Sunday evening, restock when the supply ran low. The cost averaged about three euros per mask. Over the year, this added up.
Two years ago I read, in a small skincare forum, that the sheet mask is mostly a delivery vehicle for the serum that soaks it — and that you could achieve a similar effect with a small thin cotton cloth, soaked in any hydrating serum or even just plain rose water, applied to the face for twenty minutes. I tried it on a Sunday. The result was indistinguishable from the branded sheet mask. The cost was almost nothing.

The substitute, in detail
Take a thin cotton cloth — a small face cloth, a clean handkerchief, half of a muslin cleansing cloth cut down to face size. Wet it thoroughly with any hydrating product you already own. Wring it out so it is damp but not dripping. Lay it flat on the face for twenty minutes. Remove. Press the leftover serum into the skin. Apply moisturiser as usual.
The cotton cloth is reusable. Wash it with the rest of the laundry. The serum or rose water is something you might already have in the bathroom. The total per-application cost is perhaps fifteen cents of product. The result, in my own side-by-side testing across about ten Sundays, is identical to a three-euro sheet mask.
What the sheet mask was actually selling
Convenience and ritual, mostly. The branded mask comes in its own sachet, can be opened with one hand, applied without measurement, disposed of in the bin afterwards. The substitute requires you to wet your own cloth, measure your own serum, and wash the cloth afterwards. It is fractionally more work. For some people, the extra work might be a reasonable trade.
But it is not, in any meaningful sense, a better product. The mask itself contributes nothing — it is just a thin piece of synthetic fabric that holds the serum against the face. The serum is what does the work. And the serum, decoupled from the mask, can be used in much more flexible and much more cost-effective ways.
On the small environmental question
Sheet masks are, almost without exception, single-use synthetics that end up in landfill. The reusable cotton cloth substitute is, by contrast, a single small piece of cotton that lasts for years. If you do this once a week for two years, you have prevented somewhere around a hundred sheet masks from ending up in the bin. That is not a major environmental contribution. But it is not nothing, either.
I have not bought a sheet mask in two years. The face has not suffered. The Sunday ritual has continued, in a slightly more handmade form. And the small ongoing cost of being moisturised has dropped substantially, without any change in the result.