"Even a jail sentence of ten years could have prevented WWII". Yeah, see, that's just it. The Germans, unfortunately, could not read crystal balls, and therefore had no idea that WWII were coming.
A little history lesson (you're lucky - one of my classes this semester spent quite a bit of time on interwar Germany - I'm a veritable fount of knowledge

): After the First World War, the Weimar Republic replaced the Imperial government. Weimar was a democracy, was very new (est. 1919) and followed both war and an internal revolution. Several - and I mean several, there were so many different political parties I kind of glazed over during this part of the lecture - were vying for power.
Given these facts, and the fact that Hitler's coup attempt was largely nonviolent (he had SS men with guns but they weren't used, and he was dumb - after his takeover he let Karr go), it's not surprising that Hitler didn't get a larger jail sentence. Hitler was still a leader of a political party - he still had the support of a chunk of the population (not a very large chunk - but no party had a very large chunk). It's likely that the leadership viewed his actions as somebody who's still naive and new to politics, and would calm down with time. And what were they gonna do? Arrest every party leader and every revolutionary who agitated against the party in power? The government was still very new, and still trying to find it's footing. So no, I don't think there was reason or motivation to lock Hitler up for longer than he was.
What I mean by "thoughtcrime" is that you want to lock people up because they have the ability to commit future acts. We can't lock 1920 Hitler up for the actions of 1933 Hitler. And we can't lock a petty burglar up today based on the fact that they may commit a more heinous crime tomorrow. We have to take each crime individually, otherwise it wouldn't truly be a justice system.
As an aside, I think Hitler always comes up as an example because he's so easy to hate. Our gut reaction is, if I could go back in time, yes, I'd lock him up and throw away the key. The Third Reich was a heinous crime against humanity. Which, IMO, makes these kinds of questions unfair. We can't answer objectively because of course we want Hitler to die, and any attempt to be objective makes us (or me at least) feel gross because we're defending what is easily one of the worst people who has ever lived.