Closing is not enough. By the time a reasonable, but off topic question is closed, it has accrued an answer or two, and the person who asked it gets what they wanted, even though they posted a question that isn't allowed on the site.
If we do not delete questions which are inappropriate for this site then people will understand that such questions still result in answers and reputation, and they will ask them more frequently than they already do. This will drop the signal to noise ratios, and the site would lose it's laser sharp focus. Experts would get fed up with the low signal to noise ratio and leave.
We must aggressively protect our niche, and deleting is one of the very important tools for this.
Further, if we remove deletion we're going to have to fundamentally change how the site operates. Deletion removes reputation accrued to bad questions, so removing it allows yet another way to game reputation. Deletion removes the question from google search results, and for bad questions with no answers, people would end up here via google, and see that their question isn't answered here, simply because they used the same terminology as the person who wrote the bad question. This is bad branding.
Lastly, if a question lasted long enough to get several great answers, it's quite possible that it ended up in the data dump prior to deletion. You can get that information out of the data dump. Worst case, you post a message on meta asking that a 10k user give you the content so you can repost it on your site.
The end game for Stack Exchange is that a site will exist for every reasonable question, so ultimately your request will come to pass simply due to the fact that we will have a place to migrate good, but off topic, questions.
Until then, though, it's critically important to maintain a very high signal to noise ratio, avoid rep gaming, and make sure we don't pollute search engine indices.