Why is it necessary to continue to use the apostrophe in don't?

This is a case where afaik there is no existing word spelled "dont" which is confusing. Is there any evidence that this is becoming or will become acceptable spelling? More broadly, is texting and Internet usage going to become acceptable in standard books? I see no reason for example that "u" or perhaps "U" can't replace "you" as much as seeing it in emails a couple of decades ago used to rankle.

Writing "don't" instead of "dont" is certainly not strictly "necessary". But little in language is. Rather, it's conventional. Wearing matching socks is not necessary either, but if you flout social convention without a reason that other people see as convincingly strong, you're liable to be thought of as eccentric.

Abbreviated writing is much older than texting and the internet. People have used it in note-taking or shorthand, telegraphy, and even in formal texts at some points in time (medieval scribes used a lot of abbreviations; I'd imagine to save on space, ink and time). Since non-abbreviated spellings have survived through all that, I doubt that they're going to be rendered obsolete by the modern phenomenon of texting.

Similarly, the apostrophe, despite being unnecessary, is still beloved by many people who see themselves as "sticklers" for "correct" spelling or something like that (they may describe the use of the apostrophe as "proper grammar", even though it has little to do with what linguists think of as real grammar).

Using spellings like "dont" in formal or official contexts is stigmatized as an error, and I see little signs of any recent decrease in the stigma that English users attach to "incorrect" spellings. But nobody knows for sure what will happen in the future.