I clicked through to the Guardian article and read the first page of comments - a mistake, probably. What I found depressing about most of the comments (which were the usual mixture of declarations that the language is going to the dogs, spurious rules, assertions that spelling mistakes are a sign of a decline in educational standards, and so on) was the writers' inability to construct an argument, let alone address the points being made in the article. If these are the purists, you can keep 'em.
I suspect that your experience with comments on language articles is similar. The nature of online discussion favors quick, off-the-cuff assertions. And the discussions in comments (and I include those of professional editors) quickly veer into tangents, or collect such a profusion of comments responding to something three comments back that it would be difficult to identify a thread of argument even if one existed. You won't find many locations with comments as informed and thoughtful as those at, say, Language Log.
All the same, I find a kernel of something admirable in the peeververein's resistance to argument, a kind of perverse appreciation of the democracy of language.
They are, after all, saying, "It's my language, and I have a right to use it as I please." Just so. As do the people the peevers deplore. Where they go awry is to assume that their individual aesthetic preferences are the norm.
One also has to appreciated the psychological dynamic in the response. Writing does not come naturally to us, and any skill at all in it is achived laboriously over a long time. To be told abruptly that something one has considered a rule for decades is no more than a crotchet, and a crotchet better abandoned, inevitably provokes a psychological resistance. As Dr. Johnson said, "We are more pained by ignorance than delighted by instruction."
Dealing with the purists poses difficult choices of strategy, such as whether it is preferable to go for sweet reason or a short, sharp shock.* Whatever approach you take, it would be well to stay firmly on message with some basic points of argument:
Item: Bad spellling is not a character flaw. Orthography is the great disgrace in English, rising from its bastard heritage as the offspring of two languages, and from its promiscuous encounters with additional ones. Spelling has always been a problem, and programs for its reform are and have always been farcical and doomed. It's a difficulty we all have to live with, but there is no cause to make it into more than it has to be. Just fix it's and its and move on.
Item: Lay off the Young People. There is no evidence to think that their slang is doing any more damage to the language than the slang you used in your youth and are now embarrassed to be reminded of. Slang, cant, and vogue usages rise from generation unto generation. A few of them stick in the language, but most of them are as evanescent as "23 skidoo" and "groovy."
Item: Look for evidence. It's instructive to look at Jan Freeman's analysis of Ambrose Bierce's Write It Right.** After a century, Bierce's confident assertions about usage have aged so badly that in some cases it is difficult or impossible to determine a meaningful distinction. The advantage that linguists and lexicographers have over the peevers is access to empircal evidence of how people use the language, in all its registers. They do not have to rely on mere assertion. If you want to justify your assertions about usage, you need to demonstrate that the usage you favor has been accepted and widespread over a long span, especially by reputable writers. And you have to address contrary evidence.
Item: English is not a goner. Whatever your age, it can be reliably assumed that in your youth people complained that students couldn't write worth a damn and that the language was going straight to hell. It's an old, old trope. Cicero fumed that people were no longer writing good Latin. English has done quite nicely for itself over the past half-dozen centuries, and it does not require your protection.
*My object all sublime: To let the punishment fit the crime.
**The text, without commentary, can also be found as an appendix in Theodore Bernstein's Miss Thistlebottom's Hobgoblns.