To avoid discrimination for a start
Good point. I'm doing the most part of the job to make sites accessible as much as I can. But not always 100% usable and comfortable for ALL kinds of disabled people. Need to learn these in details.
Then I'm sorry but your hypothetical client is a fool.
1. They are not hypothetical, that's real situation with real clients. I hear it from 9 clients out of 10. Even when I start talking about features for disabled people they look at me like I'm an idiot.
2. They pay for the development, they can insist or choose another company to work with. I prefer to have them stay - it's business.
In the UK
This is the main point. You live in UK and it's NORMAL for you. But hey, there's the rest of the world. I live in Ukraine. It's a poor country. We have no sound warnings for the green lights on the road crossings, no braille on product price tags or packages etc. Disabled people in our country ARE POOR. The government does not care about them. Often even families don't care much, unfortunately. This is sad. But this is reality in many countries, including all ex-USSR (and Russia to, believe me).
Are there clients out there who do not want a slice of that market?
These market in Ukraine is much, much, much smaller. I would rather say it's a micro-market. By the way, despite Ukraine is a large country and plastic cards are VERY common and popular (everyone have them) all over the country people still don't pay by card on the internet. Even on largest online stores the main (over 95% of orders) payment method is 'Cash on delivery'. This is just an example. Always remember that there is a lot of other countries outside UK and US and in many of them everything is COMPLETELY different. I saw many foreign companies coming to ukrainian or russian market and failing just because they were trying to work the way they work in their country. Always consider mentality and specifics of the country/people you are going to work with.
Who said he had to? Accessible design is not a premium add-on. It should always be part of all design projects. It's part of WordPress core.
You need to spend some extra time to double-check everything and even test (the perfect scenario). This takes time. Time means money. When the client says "let's see if we can cut off some costs" you start optimizing every step of yours. No responsiveness, no optimization for slower connections over mobile etc.
Isn't dropping support for IE6-7 a discrimination? From my point of view, making a website 100% accessible is the same as making it work in IE6-8. If it's broken and visitors can't use it properly - it's bad. If it's working and users can use the most important features, even if they don't get all the pretty stuff and extra functionality - it's ok. Making everything 100% accessible and functional in all possible scenarios for every possible user is utopia.