Pyke is a genuine forward threat now, kicking 20 goals to Mumford's 5, which is really what you want from your rucks. Pyke has evolved beyond him and is still improving, age has nothing to do with it if he isn't slowing down.
Not saying you should get ripped off, but giving Smith is an awful lot for a ruckman who hasn't met his potential, when they could go get a for half the price Currie and expect similar output.