PayPal grew by incentivizing usage. You say "bribe" to give it a negative connotation when it's not at all. Like a store giving away free samples. Or a freemium service plan. It's no different, the business is eating a cost to grow, and that technique is used a zillion times over across all commerce. PayPal was de facto spending money on marketing.
Your Clubhouse example isn't even a negative example, you're just trashing on them.
Slack grew by leveraging an advantage, like all businesses do and without exception. You might as well disqualify all founders of all businesses because they have any number of advantages, including being born with a superior mental capability (Joe Smith is a genius at math, it's totally unfair). The Slack founder advantage was earned, he had every right to leverage it. Before that he built Flickr with others and he wasn't nearly so famous then and Flickr was also a network example.
The premise you're floating is: guy that started a convenience store was able to get a loan/investment or otherwise had capital. It's unrealistic because he used an advantage someone else didn't have. Therefore not a valid business strategy.
Hotmail put a promo at the bottom of each email. Totally unrealistic that they leveraged their existing scale to grow even faster. What normal businesses can do that? (answer: most of them)