Ethics?

I don't think it's necessarily cost-effective for many places to have accessible toilets. They do it because they've decided it's the right thing do to, or because society has decided this and made a law. Nor do I yield my seat to the elderly because it's somehow cost-effective.

While you and I may consider IE8 a handicap :) it's theoretically possible to upgrade. Barriers to upgrades are usually corporate policy, very old OS versions, or low tech knowledge. All are solvable, to some extent, by action on the users part (or their IT guy). There are exceptions, but it's a tool issue on the user's side, and other tools are generally available.

True disabilities are usually not within the users, or their companies, control. They can't use their own resources (time / money) to fix the problem, or lobby someone else.

Yes, it's a tough decision what to support. My point is that the percentages of disabled users are often far below what would make financial sense to support, but often there is a need or legal requirement to support them regardless.