[RedDevil]

I'll bite.

A hybrid is not just an engine plus some electric gadgetry. It is a system.
The HSD tranny in a Prius is unlike any other and could not work in a non-hybrid.
The 2nd gen Insight (and likes) just use a CVT - but get the most out of that.

My dad bought a Jazz (=Fit) Hybrid a few weeks ago.
Aha, there is your hybrid that was not designed to be more aerodynamical than a non-hybrid, apart from some details lile a slightly smaller grill (it does not need to be as big).
The Fit Hybrid is heavier, yet much more economical than the comparable CVT trannied Fit. It is also faster.

My dad bought it because it is more pleasant to drive.
Small cars with auto trannies usually have a horrendous latency when you accelerate. CVT drums take ages to spin up. Conventional trannies need a second or more to shift down, then think again and shift again.
Not the Fit Hybrid; the electric motor pumps an extra 10 kW into the drivetrain to make them spin up immediately on acceleration. It has traction control - and it needs it. And it is smooth - no jolts, no uncontrolled RPM changes; the acceleration matches the position of the right pedal.

My car is technically identical to the Fit Hybrid.
It has a tiny 1.3 liter 8-valve 4-cylinder engine.
It has the 10kW electric motor/generator.
It does not have a flywheel. (that's the EV motor, actually).
It does not have an alternator. (again, the EV motor does that)
It has but never uses a starter engine (yet again - taking 0.6 seconds for it at most)

On light or no throttle it will go into DFCO - but then it will also start to generate electricity to recharge the hybrid battery.
Or it wil push the engine to prolong the glide while maintaining DFCO, when you still pust the throttle.
At lower speeds it can even accelerate electrically.

It cannot do that indefinitely; the state of charge will drop even if you keep driving very slow - or to be more precise, at very light throttle.
It will fire up the engine again at some point, but instead of running it very lean (and inefficient) it will open the throttle wider and recharge the battery with the excess power.

Now you may think you don't need all that when you're on the highway.
Constant speed, constant throttle, what's the use of the hybrid system then?
I'd say hook up that gauge and see how much the power actually fluctuates.
Using CC I see 50% variations in power on an almost flat and seemingly featureless motorway.
So I see the system charge and discharge somewhat, ironing out the hidden changes.
When I try to DWL I find that I have to adapt the pressure all the time to keep the speed more or less steady, like in going with the flow of traffic or keeping within 10 mph of target speed.

But even on a windless day on a perfectly flat road the hybrid system helps, by allowing the engine to be small and rev extremely low while being ready to provide the power when it is needed.

Some people here ecomod and ecodrive their cars to the extreme.
I drive carefully and as slow as conditions allow for - which usually means running it at speeds up to or above 65 mph in the right lane of a multilane highway. If there are no traffic jams.
I dodge those going the rural route - but rural is relative in this densely populated country, and it is often jammed just the same.

I don't think I do bad, given the circumstances.
I doubt anyone could beat my economy in a non-hybrid of the same size and using gasoline, driving the same route in the same conditions and speed.
Because I know how to get 90 mpg in my car, without effort.
Just not possible in these conditions.