Pretty much. Dyesubs are considered to last very long. It is possible for them to wear (I think sixty has a pic of a worn one somewhere) but that's probably after a very long time of very heavy use. And even so, I don't think it's possible for the legend to completely wear off so much as discolor slightly. Sixty might be the better person to ask though.
For the dyesub keys, they have ink permeate a short way through the keycap, so you'll have to wear through that much plastic before it becomes unreadable. For a hard plastic, like PBT, that's gonna take a while.
There's also a pic of a key completely worn through, so even a doubleshot would be without a legend. I remember it being from a medical transcriber's keyboard.
To compare with the common option #3, Pad printing, my boss's keyboard at work has almost no legends left intact. Judging from key wear, I'm sure had he these dyesub'd PBT keys, I'm sure you'd still be able to read them all just fine.
Dye sub PBT is what IBM used, and should last a lot longer than I'm planning on using the keycaps.
Also PBT is harder than ABS (and more brittle (less fracture toughness,) but that's not an issue for keycaps) so it shouldn't wear as much anyway.
Finally, if the SP PBT is anything like the IBM/Unicomp PBT, then it'll feel really nice on the fingers (which is the best part about keycaps: The keyfeel)
In short: It'll take a really long time to wear through the key legends.
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I'm pretty excited about SP's dye-sub keycaps, though I don't have doubleshots to compare. (I think I may have seen a doubleshot once, an a typewriter, but I'm not sure)
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Also, thanks for clarifying about the blank keys.
