To some degree the 7 days is arbitrary...a bit over 6 months ago posts actually had a payout period of 24-48 hours (time was extended with bigger votes coming in.) The we got a second payout one month after the first. But there is a singular time frame we do all share for our payout window.

Regarding the question around the permanence of the blockchain being mixed with the 'fleeting nature' of things here with the 7 day payout period:

The blockchain itself is the underlying record keeper/database for anything that connects with Steem (the currency). There are other applications now using this same Steem blockchain, like busy.org, chainbb.com, etc but all of these (including Steemit) are pulling from this same set of records/database. But things that having this permanence of information provides levels of accountability that can be quite useful for a social group. A cheesy example are all the 'ass-hats' that piss everyone off, get their reputation flagged down, who then goes whining about how unfar it was they got flagged.

But it makes perfect sense when you can go back and see them being flat out abusive without end. Even when they tried to edit those parts out...we can still find the original in the blockchain (even if Steemit, the front end, isn't displaying it after being replaced by the edited version.)

This aspect of things really wont change..the Steem blockchain itself is essentially the backbone of Steemit.

Now regarding the rather short period of voting...

Again, it did used to be a lot shorter time...meaning most things had about one day to get it's votes, be shown in various feeds (like trending), etc. The 7 day period still isn't terribly long, but I do prefer it this way myself. Maybe someday posts can receive votes (on an open payout)...but there is extra 'baggage' by having every single post payout remain active...which would be noticeable when the witnesses are slowed way down when running the blockchain (within the prescribed 3 second transaction times.) But since Steemit has been running, the vast majority of votes are given to posts with the first week (even when we had that second payout a month out.)

I'd figure we'd see this payout period get extended at some point down the road, but I'm sure it's going to be a long while. Personally I expect we'll see an option to pay a little steem or steem dollars to extend the payout period..just to every post would stay open.