Something for the Testers...
I have used my iPad extensively since I bought about three years ago. I have taken great care of it, and it looks just as it did the day I bought it.
I don't think wearing the screen out is anything to be concerned with. Considering "Computer Years" - which are worse than dog-years, I have to think it would not be an issue even if the screen does show wear after extensive use. Here's what I mean - I once had an iPod Touch Second Generation. If the screen showed any wear, I would not care about it. My iPad's a little different. It was priced at about $800 when I bought it, and I would hate for it to die on me even now. But I use it for a gaming device, and have spent hours and hours on it on some days, and there is zero evidence that the screen is blemished from over-use. I would think, therefor, that something else would go - such as the battery - before the screen wears out.
The battery can be replaced, but the cost of replacing the battery is likely to be more than the device would sell for by the time that happens.
I have to say - the worst thing I ever did to my iPad's screen was attempt to put those protective membranes over it. I tried and failed that with three different screen covers. I heard that the screen is coated with an "oil-phobic" finish that does wear off after a while. If so, then I would think it came off when I removed those really sticky protectors when I failed to put them on satisfactorily. To this day, there is nothing I use to protect the screen (save for the case I keep it in when it not being used), and there is no evidence that any coating came off or is worn off anywhere on the screen. If you keep the screen nice and safe, it will stay that way - no matter how much you use your iPad.
All that said, I would like to see just how durable the screen really is. On YouTube, there are videos of iPads (and other such devices) going through a "Torture Test." I would like to see a buffer, or some such thing, rubbing the screen out until it shows signs of wear.