your mechanic has done a shoddy job or just does not know about engines or wants you to come back to him every month or sooner.
A good engine overhaul should engulf derusting the whole engine cooling galleries also, completely squeaky clean. The rust/mineral deposit coating inhibits heat transfer to the water in the coolant - even a deposit half a mm thick can cause a reduction of 30% in engine cooling.
The biggest reason to keep engine temperatures up is to keep all metals happy and to keep the oil free of water condensation and toxic waste produced by blowby in normal running conditions. This alone elongates oil serviceable life too.
Any sort of engine should run on recommended temperatures normally being 90 degree celcius with proper coolant concentration with correct engine tune up and idling speed, (not the lowest idle our ullu mechanics propose) - too low idling speed causes coolant to literally stop moving in some cases.
Hence a thermostat is ideal - and the cooling fan running directly with the presence of a Tstat is useless because the radiator will only get hot when the valve opens up.