You upgrade with the resources you get in the game from either letting them build up for free or paying for them, not just by using paid upgrades. The reason you upgrade is to create resources faster, so if you invest a little in the beginning you will be ahead of the pack and be able to use your resources to upgrade higher and faster than everyone else, thus giving you the highest percentage of the rewards.
So, if you are manufacturing 400k of each item per day you will be able to upgrade 2x as fast as someone who produces only 200k, so by the time they are producing 400k you will be making 800k and so-on, this also depends a lot on your ability to make the right decisions, like upgrading the right things first. Or whether or not to use your resources on the heist. Which in my opinion is a bad idea until you are upgraded very high.
Since many people will be using their resources on the heist it is certain that if you refrain from doing it yourself that you will be able to produce much more in the long run and get a much higher take from it.
Hehe. You just explained a ponzi. Look. The only thing that matter is people paying money for upgrades. If no one pays money no one gets money.
You could have the higgest production in game. It doesnt matter. Go check if you returned what you invested. I know i havent because the reward pool split gets smaller and smaller trying to get you to invest more and more.