So what should you focus on? What OSI layer is a good starting point? It depends on your skills right?
I agree you should use as much established products as possible but you build your stack and create that unique differentiation on the market by also learning technology as you go.
It all depends on where you land with your idea to the market. If you are selling knit figures online - you are right that you should not set this up but if you rely on technology advancements you should “own your technology” knowledge as much as possible because you will need it to solve problems along the way.
Too often I see business leaders come in and say I don’t know technology - let’s buy it all from MS or Amazon. This is not the way to go for your whole business. It’s like saying can you run my business for me. Your business is only as good as your best knowledge workers.
Another aspect is regulations, your data might not be allowed to leave the premise of your nation and these self hosted solutions at least with proper patching procedures offer this guarantee.
Managed services in all its glory to hand over the problem to someone else also hands over key learnings and if you have a complex enough environment you will need to deal with it at some point and for small businesses/startups the cost of outsourcing might be too high to begin with.