It's not convincing to you because you spend a hell of a lot more time on the internet than he does. I really don't know what you expect.
I would expect him to hear the level of complaining and have one of his campaign staffers or (better yet) some other neutral party that
does spend a lot of time on the internet, and who isn't willing to blow it all off as "fake news!", to explain to him what it's all about in some depth - so that instead of a canned, nonspecific "I hope people be nice out there" that rings hollow he can make a relevant, specific rebuttal to these behaviors that proves he actually cares enough about the issue to even find out wth he's apologizing for before giving said apology.
Now strictly speaking, this doesn't require him to start going online himself at all, or cut down his TV campaigning, or anything like that. He can have someone do the research and simply give him a report. Although, for the love of god, considering it's pretty much agreed across the board that Sanders'
online popularity is largely responsible for how good of a showing he's been putting up, I should hope he might want to show a little interest in the medium...
But that aside, it's simple math, to me. Sanders
needs votes. The rabid wing of his online supporters should realize that this is the only thing that actually matters. If a sizeable number of your guy's potential voters are complaining about his online representation - I mean you can either just call them all stupid snowflakes and try to guilt-trip them into putting up with the abuse by declaring that if they don't Take It Like A Man and vote for your guy even while you treat them like dirt then they must be poor-hating secretly-conservatives who "never really cared" about issues some of them have literally dedicated their lives to advancing,
or you can do something that might actually convince them to WANT to come over to your side.
I'll tell you one thing - just as an example: I'm quite poor. And I very much do care about how poor people (like me) will pay
our medical bills. And as long as I know those things are true about myself, then when somebody tells me that I
have to support his guy because if I don't that officially means I "don't really care about" those things that I know for a fact that I do care about - see, as far as I'm concerned, that person has just proven that he doesn't know what the hell he's talking about which kills his credibility with me, period. I think that's how
most people react to the same kind of rhetorical bullying, and I'm betting that is how so many people get turned off of Sanders.