You can only run Uber with <100 engineers if you cut out a substantial amount of the systems Uber requires to actually function as a company. Goodbye Fraud, Risk, Safety, Insurance, Compliance, etc. That may be true for things like Whatsapp with a simpler feature set, but is certainly not generally true. For instance, people often underestimate the amount of ongoing engineering effort to stay up to date on changes in tax law in all the countries Uber operates in.
Usually the way these kinds of things evolve is it will start with some business requirement: "We want to launch in country <X> but our existing PSP <Y> doesn't support the country's most common payment method <Z>" (Uber operates in 71 different countries).
So you build out a system to abstract over multiple different PSPs (such as Stripe, though Stripe isn't listed in that graphic so not sure if Uber uses it at all) to unlock new business growth. Then you find out that some PSPs are cheaper than other PSPs, so you can save the business money by supporting additional PSPs. Then you find out that some PSPs are more reliable than others so you can increase availability by dynamically selecting PSPs based on availability and transaction costs etc. etc., layering on complexity over time.
All these things are actually built for very good reason (in this example, built to grow the business and reduce costs), but the reasons aren't obvious to the outside observer. But the work easily pays for itself many times over.
So could you run Uber with <100 engineers? Probably, if you were okay running in just a single country instead of 71. But that would be a very different Uber.
