Even for a serial it's cheap. In a live action superman adaptation you should see a live action superman flying imo.
But Superman Returns
used a CGI Superman for the flying scenes, and the Spider-Man movies use a CGI Spidey, and the Matrix
movies used a CGI Neo, etc. The animated Superman in the '40s serials was the exact same principle, just with less photorealistic technology. Back then, people weren't as nitpicky about the realism of their visual effects as we've become. Images that look blatantly fake to us were impressive and convincing to them.
The "flying" Captain Marvel in the serials was just a motionless life-size dummy sliding on a wire in a straight line. The animated Superman of the serials flew with much more speed, dynamism, and fluidity. I think the latter did a better job of conveying the idea of what the hero was capable of, and that's more important to me than whether it was live-action.
And what makes you think it's cheap? Animation is a laborious, time-consuming process. And good animation, as you acknowledge this was, takes even more time and talent -- and therefore more money -- than bad animation. Not to mention the added complexity (and therefore cost) of compositing cel animation with live-action footage. It was probably a lot more expensive to animate those flying sequences in the Superman serials than it was to make a dummy and slide it down a pair of wires.
I don't want to touch on modern cgi too much, I'll just say I prefer physical effects. I have no real problem with the animation, I just prefer the Captain Marvel stuff. Does anyone have any idea who did the superman animation for this? The serial was made by Columbia, but I don't think Columbia had an animation department.
I don't know how cheap or expensive the animation was. I don't buy the compositing argument. compositing an animated superman onto the frame shouldn't be harder than having to draw layouts and backgrounds for the entire sequence. It's just a matter of photographing a cell over the live action frame. It may have been costly, but let's not pretend this one character being animated is like a Fleischer short or something.
They actually wanted to do a live action flying superman, but had problems pulling it off. It was only then that they fired the flying effects guys and resorted to animation.
I prefer the Marvel effects, but I give them kudos for at least having Superman fly. Serials were made so cheaply, they could have just had the flying offscreen or take out that element completely, but they didn't.
I love that classic car. It would be an awesome batmobile.