You know, having sat on your comment here for a couple of days, I'm going to walk back my statement that a sheet can't be BIFL. 20 years is a good long time. And since I've got some sheets of my own that have been in use that long, perhaps not so unusual. And, I suppose they WOULD fit under the group definition of BIFL, namely: "Products that are well-made and durable (even if they won't last an lifetime) are accepted.". Now, having walked that back, I've thought of something else. In a high thread count sheet, the threads are smaller, so are they then LESS durable? And I don't know the answer.

But I can google "life expectancy of sheets" as well as any other person! BTW, I'll bet you there is somebody who has researched this, but I don't know of it. One of the links I found (below), said it will depend more on the quality of the original cotton, but if all things are equal, a higher thread count should yield a higher lifespan. But that's a manufacturer's website, so, grain of salt? IDK. I saw a lot of sites saying a lifespan of 2-5 years was normal. Do a search yourself. You might be interested in some of the results, and it won't take you long to get through a couple.https://www.verolinens.com/how-long-do-bed-sheets-last/