This could be totally wrong, but I have a feeling the 3rd elevator stage and beyond was intended to create a 50/50 survival situation to shorten play times. I doubt they thought players would find a way to master it.
Judging by the code, I'd say the developers thought they had a fix for a killscreen potentially happening. In general though, the features that seem to be carried over from Radar Scope are really half-assed. The game is programmed to handle level display up to 99 without glitching (and as far as I recall, any level above will just display as 99 without issues, too), but that's probably just leftover code from Radar Scope.
Either way, I think the general impression that the game was expected to end at L=4 (or 5) is false, rather I think the apparently "impossible" springs on this level might be their mistake here. It is by far the worst designed challenge in Donkey Kong, and attempts at increasing the difficulty just doesn't result in the same depth as pretty much any other challenge in the game.
I think a more correct assumption is that it's attempting to be more of a random casaulty than an impossible barrier, allowing the player to sometimes get through via sheer luck.
The game code does nothing to intentionally place springs close enough to make the gap impassable, I think it's safe to say they just input some variables and tweaked them until they reached the difficulty they wanted. So I think a more realistic situation was that the stage was tested by just having someone attempt to make it through sporadically, and when they reached a low enough succes rate, they deemed the difficulty balance "correct".
In fact, I bet that's how every stage in the game would be tested, and I doubt they ever aimed for a 0% success rate - however as an arcade developer, even in 1981, I think they knew they could expect that even if they had a 1% success rate, crazy players out there would find a way to conquer the challenge. If they really wanted the game to end, why not just have a genuine ending? Instead, if the intention is to give the players an idea that L=5 or L=6 can be reached, it only makes sense that they would occasionally allow the player to successfully beat the stage.