2004 Toyota Corolla - Decarbonize, or not to decarbonize?

I’m thinking of buying my aunt’s car.
It is a 2004 Toyota Corolla with only
21,000 miles on it.
Top speed for her in all the years she’s owned it has probably been 40 but I’d say most of it’s life it’s been driven at 25 MPH.
I’ve read both views of engine decarbonization and am now totally confused.
Can you enlighten me on this subject?

If it’s running well, don’t go looking for trouble.


Why haven’t you bought it yet ?

If it’s running fine, don’t worry about carbon build up.

Thank you!

But I would change all fluids including brake fluid.

Thank you. The car has been regularly serviced since it was bought by our local Toyota dealership.
I will check the fluids and replace or replenish as needed. Appreciate the reminder.