Increasing the post payout period would increase memory requirements for a hived node. In fact, there was a time during Steem days when posts paid out in two separate payments (7 days and 30 days), and the 30 day payment period was eliminated (for performance reasons, I've always assumed).

Also, practically speaking, most votes of economic significance are cast in the first few days, because the majority of voters with substantial stake are voting on an "attention" basis as posts appear in their feeds, on ranking lists, etc. So I don't believe that rewards would change much for posters if the reward period was increased.

In fact, some recent analysis of voting patterns done by @arcange using HiveSQL showed that most votes are cast in the first few days of the posting and this likely means that the reward period could be reduced to 3 days without causing any significant change in reward distribution. But no analysis has been done yet as to what performance benefits that might bring.

I think that the idea mentioned by @borislavzlatanov seems like a good workaround for "evergreen" content, and your idea for potentially automating this process via frontends also seems reasonable, but the frontend dev teams are all pretty busy right now, so I'm not sure where such an idea would fall vs their other priorities.

As a further aside, I think that authors are to some extent indirectly rewarded for past posts when they make later ones. This is part of a reputation building process. Authors who consistently make posts that are liked by voters often end up getting placed on automated voting lists, so in a way, they continue to be rewarded for those earlier posts. This is part of why long term posters often achieve some of the highest post rewards.