My '63 Gretsch 6070 has telescopic endpin. Still, if I took it off it really won't fit in a case unless it's for a double bass. Huge!
How common could it be to play a big hollowbody as an upright? I've seen a couple of pics around, though.
Well, Leo was beguiled by the then-typical playing mode of guitarists and banjo players in using their thumbs. That begat the tug bar, widely misunderstood by those who fail to study history.
I think it likely that many others assumed that bass should be played vertically, like an upright bass. Doesn't surprise me to see various endpin arrangements.
And let's not forget that the manufacturer is often ahead of the practitioner. It's not unreasonable to think that many bass players who tried electric basses made their own reflexive assumption that they should play it upright with an endpin. I'm confident that there were requests for endpin devices.