I'll note our own pg says he doesn't really need to understand almost anything about a company, he just needs to eyeball their growth rate to know everything he needs to about a company. And, if you read through the public YC training materials, they hammer this point home, very, very hard for their founders. Weekly compounding growth.
To respond to some of your other questions, I don't really believe in retaining earnings for a company this size -- returns should be deployed somehow, and this deployment should be planned for -- they have certainly not found the end of the list of things they could do with money inside the company that beats some notional outside-the-company returns yet.
I don't know how you would propose to mitigate risk more effectively than growing aggressively, but I would be interested in your perspective -- rapid growth brings its own risks, yes. But it does a few things - it provides an incontrovertible early feedback loop on your continued ability to have product market fit - and it increases your power as you grow your network of stakeholders - and it increases your economic power in that network as you have greater revenue.
In general I'd rather be sitting in the seat of being the largest most influential company in a niche, and being able to acquire or squash competitors (or expand) than finding out I'm up against a deep pocketed competitor with a vastly larger customer portfolio, and if you're starting out organically from scratch the only way to get to that large/influential spot is to put the gas on very fast while you target a small niche, unless you have some extraordinarily special sauce that isn't replicable.
My experience is that generally people are better at thinking about risks from doing things -- risks from growing for instance, risks from taking on capital -- they aren't so good at thinking about risks from inaction -- from doing the same thing they do now.
Finally, I'd say most businesses are vulnerable to well funded competitors; usually though the business they're in isn't juicy enough to really get say the Goldman special situations group interested. You'd better believe if you have juicy at-scale real world returns available to your business then very intelligent very wealthy people are already thinking about how to carve up / acquire those returns.