Yeah, they might change it. Or at least add some more interesting ways to accumulate merits.
I have just played with some values in a spreadsheet, and I can see to maintain rating 5 you need to accumulate 5340 each week.
If you do that from the start you would reach rating 5 by end of week 4. I think the thing that throws it off is the extra effort to get from rating 4 to 5 earlier. You end up with one week where you accumulate a lot more merits (I think it was between 7000 and 8000) for me. The effect this has when it is no longer part of the carry over takes you by surprise.
I doubt I will attempt to keep rating 5 much longer, it has been useful to accumulate extra cash (made 19M in about 90mins at a combat zone, due to the all the faction bonuses and 200% rating 5 bonus), but I am reaching the point where I don't really need the extra credits as much. My plan is to get an Anaconda, kit it out a little, attempting to keep the long jump range and go explore a bit. You can't really help in power play and explore at the same time.
This week is also a little harder with my time being split with 1.4 beta.