Why exactly do you think this would solve the problem? In a world where most (all?) patents being used for trolling are trivial and/or ignores prior art already. Why won't trolls just repatent something with different wording after it expires? This is already common in pharmaceuticals where some drugs have been under monopoly for way over the normal 20 years but under "different" patents. This is only not too common in software, yet, because the industry is young.
So wouldn't reducing length just worsen the problem by shifting even more power into the hand of incumbents who can afford to refill "different" patents more often than startups? Trolls would just use newer crap patents anyway.
So the answer is to deal with trolls not eliminate patents. Having a shorter patent grant is about addressing issues like Amazon 1-click. Amazon is not a patent troll, but having a 20 year patent on 1-click is absurd.
There are a lot of patent trolls outside software. Being against software patents and being against patent trolls are different issues with different solutions. I reject that dispensing with patents altogether is the best choice.
So wouldn't reducing length just worsen the problem by shifting even more power into the hand of incumbents who can afford to refill "different" patents more often than startups? Trolls would just use newer crap patents anyway.