On the other hand...
In my formative years I - and I think many people - really would read the programme notes (as the news article points out!), treating a concert as part of an education and getting in the mind set for what I was about to listen to... possibly glancing at the title / nature of each movement as things move along, often reading the notes for the second half during the interval... I can see someone being wrong-footed and even disturbed by readying themselves to listen to Bach and then hearing Scriabin or whatever. You are, after all, settling down to 30 minutes or more of close listening - though I, personally, wouldn't write to complain if the programme changed!
The classical audience is very varied. I've seen people with whole scores propped on their knees, and no end of ladies in fur coats snozzing happily, and quite a few "when I was young and went to classical concerts everyone was old" listeners. Are some more "up tight" then you might find in a jazz concert? gosh, yes, quite possibly, who'd have thought! They also dance less than you might find in a techo-rave. Also less talking, getting up for beers etc. Yes it's all music, but different kinds of gigs can be quite different experiences - even within jazz... youtube is full of videos of nicely dressed people sitting and listening to music that, in the 40s, would fill huge dance-halls with thousands of crazy dancers!
Also, this was a sell-out concert (hardly dwindling) and got a few gripes, just how bad is that (we all know journalists have to fill column inches etc...)? "upset audience"? really? Ask Igor Stravinsky what an upset audience really (or apocryphally) looks like!
Yes, I personally wouldn't have complained, indeed, a change in the order of the programme may be taken is part of the conversation about the music... that's me. Others are different... and really I see no good reason, from this, for folks to slagg off a whole demographic of music audience members!