"You play your best while they are your best. You don't replace them now because they might not be the best in two years time...you don't pick your current team on the projected form over two years...you pick the best team and gradually replace injured and retired players when needed...while also not overpaying your stalwarts...it's called responsible player management...like having some of your stars play in Japan...thinking outside the box and keeping the opposition guessing..."
I disagree.
The majority of the older guys will be just about hanging in when the WC comes.
So I would have started right now in picking my team that I expect to play the WC. And. I would have the old guys warming the bench. And, for the two AB games and the French game this year, I might start some of the older guys(Eben, Pollard, Mostert, PSDT, Kriel...maybe Willie). But for all of the rest of the games, and all the games next year, I would be starting my younger WC team and have the older guys come off the bench. And if the some of the older guys stop performing off of the bench then I'd be replacing them.
The advantage being that you are saving the older guys while keeping them involved and you are working out what your best starting team is and getting them used to starting and playing together.
Every other team in the world, including the teams we only beat by 1 point in the WC, use this approach because it's obviously the correct way to do things. Play your best team because synergy is EVERYTHING, always.