More than that, supporting old hardware costs development time and therefore money, because they have to pay developers for their time.
DE has obviously already done the math and made the hard decision to cut support of 10+ year old hardware because the cost of supporting it was far outweighing the benefit of retaining players stuck with that hardware. They didn't just wake up one morning and go "lol let's cut DX9 support to make things easier, who cares if anyone is still using it". They knew some players would be left out. That's why they gave over 6 months of advance warning.
I'd bet it isn't anywhere remotely close to 20% of the players though. Probably more like the example of Linux support for Planetary Annihilation above... less than 1% of the playerbase with ancient hardware taking way more than 1% of developer time to continue supporting