Hi Kuma,
Thanks for the question. A few notes:
First, in the 2017 edition of the LRB, this problem is now on page 247. If anyone is using a version of the LRB where this is NOT on page 247, you are using a
very old edition, and you should I've added hundreds of pages of new, up-to-date info and expanded discussions. I typically discuss the changes for each edition yearly on our blog (example:
http://blog.powerscore.com/lsat/the-201 ... ns-part-ii), so if you need some info, let me know.
Second, let's look at this answer and isolate what it is saying, which is that some institutions lose power more slowly than others, regardless of type. If you were in a conversation with the Speaker here, and you made this statement to him or her, what would the response be? Would the author immediately surrender, or would the author have a response?
As an initial response, the Speaker would probably say that due to the length of time involved ("decades or even centuries"), differing rates of power erosion are not unexpected (and are perhaps implied by the reference to "gradual erosion"). Additionally, as a basic principle, these differing rates are compatible with the idea of ultimately needing to act responsibly to retain said power. In other words, just because some institutions have inherently differing speeds of erosion doesn't mean that the general principle in play here is being undermined.
The next part of the Speaker's response would address the idea that some non-socially responsible institutions might have power that erodes more slowly than socially responsible institutions, and whether that creates an issue here with the necessary condition. It does not, primarily because the speaker's conclusion is predicated on a broad enough principle to allow for a lot of variation and possibilities inside the principle. For example, although a particular non-socially responsible institution might have a very slow power erosion, the author would still say that the erosion would be even slower if they acted more socially responsible. And, to the comparison point of a non-socially responsible institution having a slower rate of erosion than a socially responsible institution, the response would be that such a case still doesn't disprove that to retain power for as long as possible, a particular institution must act responsibly (especially because factors beyond the control of any institution might affect their individual situation).
An analogy might help shed further light on that last point. Let's imagine that this was about money, and the conclusion was: If a person wishes to retain as much money as possible, he or she must spend it responsibly." That would make answer choice (C) something along the lines of "the money (or savings) of some people erodes more slowly than the money of others, whether they are responsible or not." Here, it's easier to see that there could be many reasons for why this would happen, including how much their job paid, where they lived and how much it cost, and even how much money their family gave them or whether they won the lottery at some point. And, despite these many different cases, they still don't undermine the general rule that you should be responsible when spending money.
Please let me know if that helps. Thanks!