Course you’re leaving out NSA + other government monitoring systems, and ignoring the fact that Twitter stores all its tweets in UTF-8: one tweet could actually (although very unlikely) be 560 bytes.
Daniel, I don't think this is the problem you make it out to be. Yes, Netscape 4 is a nightmare, but it's such a nightmare that it's just not worth catering for. NN7 (well, 7.1+) is fairly easy to cope for if you need to, but in my experience the user numbers are so low as to again make it not worth bothering with. Beyond that (NS8,9) they're just Firefox 1.5/2 at the core so there's no extra burden in supporting them. Well, assuming you support Firefox :)
As for being able to download them, well, if people who weren't developers were doing that then we'd see it in the stats, and we don't.