The ending is way too sudden.

I agree with you completely. Like you As a watcher who always sides with the small-folks i would like to see a man like Mr. Taylor gets what he deserves. But i also thinks in reality that wouldn't happened and he will "weasel" out of it and his army of lawyers will get him out of it clean.

I think that the reason i wanted to see him ripped to pieces is that i know that there is 1000s Taylors walking free everyday committing crimes against billions of people.

JUST watched it for the 1st time and I'm 54! Anyway, yes! I was like....that's it! No comeuppance for Taylor? It was just over....period!

May I beg to differ?

It was way back in the old days, with the 'code'. Had Taylor won, bad story, boring; in any case.
Had Taylor been sent to prison, Smith reinstated? Then we'd be arguing about 'dreamer!' since the USA hasn't been like Smith being celebrated as a hero, with policies changing.

Even back then, they, and Capra, knew too well that the Taylors of this world will - in the end - always be on the winning side. Therefore, Taylor couldn't be shown as loser. He probably wasn't, would have his layers to get out unscathed.
I rather see it as a melancholic movie: A sympathetic Mr. Smith, an out-of-this-world person, becomes Senator. Deservedly, in the moral sense. And falls to become the prey of corrupted politicians. In the end he prevails in his single situation, yes.
And that is what was demonstrated so cheerfully.
The viewer will know that the subsequent 99 cases won't come as far, and the Taylors will win.

Until in our days. We just had a 'nice' campaign, with Trump winning. Has the established camp done any better, morally, than the Taylors of those days? Does Trump have any traits similar to Jeff Smith?
Neither. So the somehow open ending makes quite some sense to me.

Hello, Dead-thread! Nevertheless, I reply.

I like the ending as-is. It wraps up the story and while we don't get all the answers in detail, I think we have them.

1) What happened to Taylor?
With Paine turning back from evil to his better nature, I think the truth would out. Taylor sent messages to wipe out the rival presses, but he can't do that level of nastiness forever. The 24 hours necessary for Jefferson to collapse, sure, he can muzzle a state, but it would be impossible to keep doing that forever before the police - though some were doubtless in Taylor's pocket - would intervene. Furthermore, the fact is that the dam was graft, and Paine would have proof of that in documents and correspondence. They were sneaking it through, so Paine shining a light on it would blow the dam up - so to speak. All this is to say that Taylor and his machine would be broken. Taylor would face charges at least. Even if he didn't go to jail, his machine - a shadow operation relying on stealth - would be revealed and he wouldn't be able to turn the screws as well as he once could. Between that and Jefferson Smith's influence, you bet that Taylor isn't in a good spot.

2) What was Smith's reaction?
Happiness. He probably regained respect for Paine, was glad that Taylor and his machine were broken, and most delighted that Clarissa was in love with him. The thing is, seeing Smith happy at misfortune - even the misfortune of Taylor - might not be pleasant. It would be nice to see a final scene with him and Clarissa getting a smooch in, but that's about the only thing I'd want to see. I don't really need his response to victory.

As for Stewart's performance, it's incredible, and he is (as always) a sensation, but I don't think it's his best. His best performance might be Vertigo, for my money, anyway. Scotty is a knotted, complex character and Stewart goes to far more uncomfortable depths with that role. Smith is much more straightforward.