Instead of doing that, they probably dropped a bit more than a thousand dollars on a box, and are probably saving thousands in costs per year. This is money coming out of someone's pocket.
This site is here, and it's a charity, being provided free of cost, to you. Who cares if HN is down for a few hours? Seriously? Has anyone been hurt because of this, yet?
There is a very strong bias to everything YC.
The HN community has also outgrown the software HN was built on you can see this in threads like: https://news.ycombinator.com/item?id=7051091
but even that thread is an extreme many front page items that gain traction are hard to go through because of things like lack of foldable comments. Other things that are extremely noticeable are expiring links which pg has said he doesn't think are important enough to fix. There are many small UI issues that won't be fixed for the community.
Feels more like a community with some YC marketing aspects to me.
Credit to http://alexander.kirk.at/js/hackernews-collapsible-threads-v... which I referenced for dealing with the structure.
Hard to quantify "harm".
But I will say that anything that is a habit, when broken, opens you up to the possibility of being exposed to a new habit "the addiction".
Along those lines 1 day of downtime probably isn't going to shift attention much. But an extreme, 2 weeks, would certainly break some addictions as people would fork to a new (I don't know insert some french or latin word here!) So who is to say where that slippery slope of is? (Somewhere between 1 minute and 2 weeks).
 Reason that I have heard that Starbucks renovates restaurants at greater expense and keeps them open at least some hours. Because people's habits are mercurial.
eg. If there is a hardware failure, can the site still remain operational? Of course resources cost money, but you could have 2 cheaper/smaller servers load balanced for around the price of 1 pricier/bigger server.