I must say that, to me, the most logical solution seems to be _ignore that crap already!_ One of the good things about the SE system is that old, ignored questions are forgotten as long as they're left alone. Yes, some posts should be deleted because they may (and this has never been proven, but let's just say it's true for the sake of argument) serve as doorways for more crap. I might point to crappy question X and take it as evidence that my crappy question Y belongs here. Perhaps. 

However, old crap is not really harmful. In most cases, it will never be seen. So, sure, deleting old questions with no answers or no upvotes or whatever might clean some bad stuff out. But is that worth losing even a single good question? As long as the automated systems can delete the good along with the bad, I see no point in it. Why not simply ignore them and move on? Why should we bother so much about deleting old stuff when we can be doing more useful things like answering new Qs?

I realize you are not advocating focusing on the old and ignoring the new. What I'm saying is that I don't see any great benefit from deleting old ignored Qs. Closing them, perhaps. But deleting? Why? 

Anyway, if the community really feels strongly enough about this, at the very least the time limit should be way more than 90 days. You never know when a question might get a great new answer, what makes you think that it will only be relevant for 90 days. What if I have the same problem 5 years later, solve it and then post an answer. The next person to come along will now have a solution and that would have been impossible if the question had been deleted. 

So, personally, I would not even try to delete more than the system already does. If, however, you all feel that we must, then delete questions that have <=0 score and 0 upvoted answers **and are at least a year (or maybe two or three) old**.