Or, just because you understand the underlying technology doesn't mean you understand the market, and your previous experience will get in the way of actually understanding the market.
If you want to go down the tech stack, though, you might be successful because you're part of the market for that tech.
Exactly. When you go down the stack like Amazon did with AWS, you know your customer (often you). You also know the key pain points and gaps in the market. This is especially so in "as a service" markets.
If you want to go down the tech stack, though, you might be successful because you're part of the market for that tech.