Well really if you're still getting lots of heat from the heating system then it's fine to leave the thermostat in there.To me that vehicle is probably due for a coolant change , plus checking the rad for any leaks and the rad hoses for extreme softness or cracks or leaks, also basically draining the rad via a valve and then adding water to help flush out the old coolant, plus changing the thermostat, then adding fresh premixed coolant , testing the coolant with a proper anti-freeze tester and topping it off properly.
