Yeah, that's the argument people keep using.
But, if you have 100 people, and you give 90 people gold, except the 10 that use IE, I bet you that those 10 will switch to get the gold.
XenForo is a modern product, it's a modern world. And it's pretty freaking gold.
I've completely stopped supporting internet explorer, if someone complaints a site doesn't work the solution is simple: Install a free cross operating system and more modern browser like Chrome for example, and enjoy the net like it's intended. Rather than sticking to a shit product because of others having the same fear of chance.
The same with bandwidth caps.
The telecom providers say that only x % is using less than 100 mbyte month, but the reason most likely is because they're afraid of bills.
In the Netherlands we shows telecom and isp that we go over our limits, change providers, pay with our wallet, and the majority has 24mbit internet or better, even 50k households with 1gbit, and nobody has a traffic limit.
If we accept caps, throttling, and things like supporting outdated crap computers, browsers and what not .. they will continue to cap you, continue to use win98, continue to use ie6, etc.
Use what you want, when you want it, because it's available. Let someone else have a bandwidth cap, a shit browser, and a poor end-user experience.
I never understood the reasoning of 'we can not upgrade'.
But after hearing the same arguments for 10+ years on the Internet, I might just not be listening to it anymore either.
Nothing personal against you - but I would just advise to your users : The reason is because your browser doesn't support the basics of html/css/javascript. And it's a website. To load a web site in 2011, get a browser that does.
If you are afraid it's costing you users, why not use phpBB from 3 years ago?
I think an admin lingering on that 20% should realize that supporting outdated stuff like Internet explorer will lose more users than sites that understand it's 2011.