No, not arrogance, but also not a real-life conversation either. What Dilbert is saying is both "no one uses the data anyway (so none of it is necessary)" and "if you want to remove the unnecessary, tell me what 'unnecessary' means." That's been my experience: the only criteria management knows is "disk full," but there is no retention policy to define the storage needs.
This reminds me of a Bill Hicks gag....
'Hicks, how come you're not working.'
I'd go, 'There's nothing to do.'
'Well, you pretend like you're working.'
'Well, why don't you pretend I'm working? Yeah, you get paid more than me, you fantasise. Pretend I'm mopping. Knock yourself out. I'll pretend they're buying stuff; we can close up. I'm the boss now, you're fired.
Yes, that is exactly the point of the strip. Dilbert is unhappy because first he spends his days building and maintaining a d-base that is never used as the basis for decision making, as PHB (in blissful ignorance) relies on Business School (BS) anecdotes or stuff he heard on the golf course instead of data.
Based on his experience Dilbert reaches the valid conclusion that all of the data is unnecessary, as it is never used. That does not imply that Dilbert thinks that the d-base does not contain potentially useful data.
PHB is making a perfectly legitimate request. Arrogant and cynical Dilbert feels the need to again express how much he hates his job. Then PHB loses his way. Having caught PHB in his trap Dilbert then makes a cheap witticism that all engineers can relate to. Hilarious... and sad sad Dilbert who is probably much unhappier about his job then PHB.