> Why would they care if a number they don't what get disabled?

If you gave it as a security contact number to a financial institution you’d care if it was disabled.

Especially since the number will generally be recycled and given to someone else ...

Eventually. Old numbers are held out of use for, I think, five years before being reissued specifically to address security and privacy issues like this

When I had a prepaid phone you had to buy minutes once a year at minimum otherwise you lost your existing minutes and your number was disabled.