I just think it's a lot of assuming. I think Murphy is a safe hire, sure. He's probably not getting a substantial salary. We don't even know how long he wants to stay in baseball. I also think the Brewers feel he'll be a successful manager, why else would they hire him? The, "aw shucks, let the old man do it a couple years" idea doesn't really work for me. So they think he can do the job. I just don't know why any team would hire a guy with the thought they'll replace him in 3 or 4 years. Just hire the guy you think will do a good job from the start, no timeline.
Also, 70 isn't some death sentence, Bob Uecker is pushing 90 and I can't find anything online about if he's done yet. Murphy could coach well into his early 70s if he's physically and mentally capable. Dusty Baker did it, I don't know if everyone would agree he's "the best manager" but we don't even know how Murphy will do.
My only point was, as I said in my post, I think it will come down to his performance as manager more than any other factor that determines how long he keeps the job.
TLDR - I think Weeks is Plan B in case Murphy's performance is a failure. If Murphy is doing a good job then there's no expiration date on his tenure.