The above mentioned £20k investment.

To be honest, you need to be flying a lot to make any real use of it. Even then, apart from the first wing at LHR, and maybe a handful of lounges on the network, unless you're flying solely Y or PE, you're going to get nothing much from it.
Sure there's many here who get a gold card and then fly mostly in CW or have a couple of trips or flights on top of things.
You could argue then that you could be paying the equivalent of £1000s for a QF LAX visit etc. on those limit flights.

Silver, at least at present, doesn't get you that many fewer benefits.

Are the cards still going to give bonus Avios? Or just the token 1500, 2500 or whatever has been mentioned somewhere.