Your premise is invalid: most startups are not valued based on a number of users. They're valued based on future earnings potential. High number of users is related to earnings potential but it's not the only or even the most important factor. The important factor is: do investors believe you'll be able to make money off of those users.
Another invalid assumption you made is that valuation is equal investment. The amount of money invested in a company is only a fraction of valuation. The difference is ROI that investors hope to get.
Furthermore, you assume that there is a left-over investment money. Companies raise money because they have to pay for salaries, servers etc. which are more essential to success that number of people who show up on your website.
Your timeline is inverted. Even if investment are based on number of users, you could only pay users after you've received it, which defeats the purpose.
Assuming that investors are not stupid, they would discount the valuation if they knew about that scheme. It amounts to nothing more than scamming the investors using their own money and clearly you can't hide it.
You're trying to create a perverted incentive scheme that is economically untenable and defeats the very reason business exists: to provide services that people are willing to pay for. If you have to pay your users, you don't have a business.