I am not yet accepting your claim that the switch is stuck in the "on" position, unless you investigate further and measure functions and resistances (read below).
Open the mic, and observe whether the -10dB stalk is structurally intact. They sometimes break.
If is not broken, check that the stalk's rectangular contact, pressed into the opposite end of its actuator depression, touches and bridges the two spring-loaded contacts when you activate the switch function (upper position). If it mechanically touches, measure the resistance between the two spring contacts. If that shows open, clean the contacts on the mic amp and on the stalk. If it reads zero ohms, it's time to send it in.