I don't know why they would need to change the rules every 5 years if the system works (everyone will have there own opinion as to whether it work of couse), with a few exceptions, to keep it fresh I suppose would be towards the top of the list, and perhaps to simply keep up with the time culturally. I have heard people say they feel 6th edition is more fast-paced/cinematic for example, which seems to fit the times in terms of cinema and television.
It doesn't stop them releasing new starter boxes or shouldn't in my opinion, new people are getting into the hobby all the time and a new starter box with the same 6th edition rules set would sell just as well (i'd buy just for the cheap miniatures), the only thing they would lose out on would be the rulebook, they could probably still re-vamp individual codicies within the same rule-set also.
As mentioned it would free up time and money to concentrate on areas needing some love and perhaps develop new games/systems.
I think it would be a good thing if they continued to release new starter sets and codicies from time to time.