It's the nature of technology. It moves ever onward.

It's not an issue that people want to game on an older version of Windows but we do expect Steam to be updated and updating it requires stuff that doesn't work in those versions of Windows.

If Valve wants to make a version of Steam for each OS that their main branch doesn't support, that's mostly a matter that's up to them. I'm not gonna say they shouldn't do it, but neither am I invested enough in old operating systems to care much about trying to get them to do it.