Some Steam user says that users who use Windows XP buys nothing and to save Steam client workable on Windows XP by unprofitable.

Let's calculate! Steam statistics have reported at the last time about 0.35% Windows XP users. Total registered user in Steam: 125 millions (2017 year info, so it's higher)
125 000 000 x 0.0035 = 437500 Windows XP users.
Even if each of them spend only 1$ per year and Valve gets only 15% that's enough to hire an 16 developers to solve that elementary issue.

Also don't forget that the most of Windows XP users have a machines with other OS. Therefore users spend more then 1$. And they really may start think to change the side.