Yitay, I had that same question. Add that to my list. Also, if it was dangerous to be outside, as it probably was on any other holiday, people wouldn’t go out. You don’t have to make such a Takkana.
Why couldn’t they establish a much better Minhag, to stay in Beis Medrash all night just light Shavuos? The Gra advises to learn at home all the time, so I gather that people were able to do so. I also know that even if people didn’t have the whole Shas, they had Seforim. This “trick” of not learning is the last thing you’d come up with if you simply want people to stay home. In fact, if the Minhag would simply be to Daven Beyechidus, people would stay home and learn there. And again, this Minhag actually caused us trouble rather than the other way around.
One more question I have is, aren’t we talking about a ghetto? What are Goyim doing there?
Why was this Minhag in Hungary, Poland and Russia? The conversation of trying to figure out where this comes from goes back at least 200 years, since the Chasam Sofer quotes his Rebbe on this topic (and guess if he said it is just a way to stay home).
It’s one thing to think up thoeries, and it’s a lot of fun. But taking that as the reason, so much so that you can uproot the whole Minhag, is going very far. Look, if you don’t have this Minhag (anymore), I wouldn’t advise you to take it on, but mocking, or being bothered by an ancient Minhag Yisroel is unacceptable.
This should be put in perspective with the fact that much of what is being done instead of learning (playing chess, taking it easy) is often praised by some of the same people who are against Nittul as worthwhile endeavores.