I absolutely support the second solution : "I think we can propose a new way to compute the reward system: Imagine the possibility to be rewarded for a post indifenitely. That is, eliminating the 7 days limit.".
I don't think it's easy to implement, it will certainly require a profound rethink of the whole system but the truth is, Steemit functions as a blog, which "depreciates content" as if was news. A lot of people post fitting content such as technical analysis on crypto. But many other people prefer to post content that does not lose value after 7 days (if ever). It is quite frustrating even for the reader (let alone for the author) to stumble upon a beautiful gem such as this post of @zipporah and not be able to reward the author for the pure joy of reading because ... the post is 24 days old !
All the while a lot of energy is being spent left and right by @berniesanders and grumpy cat fighting the likes of haejin and sweetsssj and random beginners unaware of some arbitrary rules ...