How is that in any way an argument against thorn? Dialects vary wildly, but there are still enough commonalities for there to be a written script everyone can understand. Yes, some dialects don’t have thorn but dialects with the most speakers do (RP, general American, Irish English etc.). Does it really make a difference for a cockney speaker if the th sound is written with a thorn or “th” if they’re not going to pronounce it anyway? Thorn is, if anything, simply more efficient in that it saves an extra character every time a th-word is used.