Has any aircraft ever been designed such that it could descend safely with no control input?
Broadly speaking most (if not all) light GA aircraft can "glide safely with no control input" - aircraft are generally designed to have positive dynamic stability, such that they will return to a stable equilibrium condition (e.g. "level cruise flight") in the face of most modest upsets. Once configured for cruise flight they can maintain it with little input from the pilot (and if equipped with even a basic autopilot "little input" can often be reduced to "no input" for extended periods of time).
Whether or not the engine is producing power is largely irrelevant here, save for the fact that if the engine isn't producing power you will eventually be descending.

Positive stability alone will not make aviation "accessible to the masses" however, nor will autopilots: As with driving a car or riding a motorcycle there are certain "aeronautical decision-making" skills which a pilot must possess in order to safely fly (and land) an aircraft when everything goes right. If you introduce problems (like engine or instrument failures) the need for a real live pilot becomes even more critical: technology can not yet replace the critical decision making capabilities of a trained human mind.



Particularly in regard to your question about a default flight mode of a controlled descent, even a simple autopilot can already do what you describe: Planes will continue to fly the last autopilot-commanded heading or route until they run out of fuel, at which point they'll start descending (while still attempting to fly the programmed profile).
What they cannot do is select a suitable landing site, update their flight profile, and fly themselves to a safe landing. For that to happen without human intervention requires a huge amount of luck.

In a particularly famous example of this kind of luck a Piper Comanche "landed" itself in a field after running out of fuel, the pilot being unconscious at the time due to carbon monoxide poisoning. The pilot survived and in fact literally walked away from the accident scene, and the aircraft in question is, as best I can determine, still flying.