I think it's a case of using what you've got properly. Ah Mau is no world beater, but he is a solid, reliable first grade prop. He might not be flashy, but he does his fair share and generally takes the team forward.
Matty Prior is a similar type and was woefully used with us (putting him in the centres did my head in!). I remember him being singled out for a shiteload of abuse in here, but he went to the Sharks and (by being used properly) was able to help them win a premiership.
Unless you are the Broncos or Roosters, it just isn't possible to have a team full of superstars; you need the Ah Mau's to fill the gaps. If you want to supplement him with a decent prop like Walmsley, that's great... but replacing him to me seems wasteful.
P.s. Ah Mau would be far better for keeping the rising props, simply because he is (presumably) on far less than what it would cost to get Walmsley.