Spending a lot of money does not guarantee a flag.
It does typically guarantee that at some point, the team will be handcuffed by those large contracts and will have to reboot. Without those prospects, proven or not, the reboot just takes that much longer.
Teams can easily win championships without spending $300M + on a player. It starts with a strong farm system. You know that. I'm not saying that a team should never trade away a prospect.