I'm not aware of any mechanical throttle system designed to fail-safe to idle. Indeed, on balance I would prefer an engine to remain at the previous power setting. At least it would probably give you more chance of reaching a suitable destination to attempt recovery.
In all of the single engined aircraft that I've flown - piston, turboprop and jet - the teaching for a 'stuck throttle' was to do your best to home towards a suitable airfield before completing a modified forced landing pattern. At a suitable point the engine would invariably be shutdown.