I agree with you that it couldn't survive today, but I often wonder why. If I had access to stumbleupon as it was, I would absolutely be using it - but when I try to think about how to reimplement it there are a couple sticking points that I don't have any solutions to:
- Engagement: SU lived and died on it's users, a paragon of the crowdsourced model. For it to work you'd have to have it pull enough interesting people from the mire to function
- Gaming the system: One of the things that made SU great was that there wasn't so much goddamn SEO out there. If you 'stumbled' on a thing, it was because it was interesting, engaging, funny, or otherwise *actually valuable*. These days, I can't imagine a successful platform *not* getting beleaguered by the SEO vultures.