you know that the content will last beyond 7 days, it would also be an incentive for quality.

That's a good point. I've come across good content that is more than 7 days old which I'd like to support but "likes" aren't registered and the author won't benefit (whereas I think YouTube still rewards creators years after the initial publishing). This would be a great incentive for increasing quality and that quality would subsequently attract more traffic from Search Engines.

There are so many challenges with regards to maintaining quality. Using me as an example, I know that every post has a minimum amount that it will be voted due to the auto-votes & curation trail support. So (in many ways similar to a voting service), I could write a "lazy post" every day and hope that my current supporters don't lose interest. But that's not me and I take pride in everything I write and everything that I share (including the comments themselves)... which is why that "base level" support exists and I don't post something every day.

Linking back to your other comment, it ultimately comes down to motivation for writing. If users are solely motivated by the rewards, then I believe that comes through in their activity which will probably have a detrimental affect on their rewards. For those that take pride in everything that they produce, it could well be a "slower burn" but once they're alight, they'll burn long and strong.