[RedDevil]

[hybrid vs. non-hybrid highway cruising]
What I see on the highway is that even cruising at a more or less constant speed the engine load differs quite a lot.
There's packs of trees blocking the side wind, then open space.
There is the odd canal or crossroad to bridge every mile or so.

I can see the assist needle jump up as the engine load jumps to 80-90% but the revs stay the same (CVT maxed out in top 'gear'). Then descending it starts to generate, then DFCO and give assist to maintain speed. The hybrid system does kick in even if you hardly notice why.

Even so on the highway it does not make a big difference to having the same engine setup but no hybrid support.
I got to find out as I once blew the fuse to the electronics that govern the battery monitoring system. So the system shut off the battery, and I had to do with just the ICE.
On the highway FE was normal, maybe slightly higher but no big difference.
But entering the highway was a drag. The engine really had to rev high to compensate for not having support. It wasted a lot more fuel than I was used to. Leaving the highway it wasted all its speed to brake dust; it hardly slowed down at all unless I used the brakes. It does DFCO but no generating of course.
In the city it was just wasting fuel and slow to get off the lights.
Despite my best efforts it used about 25% more than usual on my morning commute.

The point is, without hybrid support it is no fun to drive; you'd want a bigger engine.