Very few people using it seems the very definition of withering. Yes, a lot of sites and blog software etc. still default to offering RSS feeds. (Which I take advantage of from time to time and it's presumably used behind the scenes for various purposes.) But that's hardly a ringing endorsement of the role of RSS in today's world.

Why do you care if other people use it as long as it is available?

Because when things aren't used, organizations such as, say, Google tend to stop supporting them over time.

It’s been over a decade since Google abandoned reader. RSS is just a check box in most CMSs.

As ghaff mentioned, it’s the very definition. COBOL is still used by some old banks, but you’d still say it’s popularity has “withered.”