There is at least one reason for Sony to improve the already sold products: To sell again to the same customers.
If they leave you feeling that they don't give a damn, you might still buy their product if you are happy with it, but if you are in doubt, they are out. The 505 is a great e-reader, but I wouldn't say it is the only good one or even the best.
In my case, the only reason why I would like them to produce some new firmware is that it will permit to hack the 505 without risk. Apart from that, I don't really need anything that is not already there. In that sense, I would be satisfied with the release of the original firmware. Cost to Sony, 0.
One of the reasons Sony sells more than Nintendo in many places is that the playstation can be easily hacked (hell, it has a hollow slot to place the hacking chip). That is one of the reasons people are so faithful to their PRS 500. Let the 505 be hacked and you'll even get some of these.
Considering all this hacks won't change the durability of the reader to the longer, but might make reading nicer and therefor make the product be used more, you'll even increase the product rotation times. All this with minor or no changes in the firmware.
They should have released something right away with the reader. Their mistake.
My humble opinion, of course