There are a lot of things that are trivial to build. That's often not the question at a startup. At a startup, it's "what is the most valuable to build". Implicit in this is often, "what's unique to my product that I must build".
If I can take X to drop in a 3rd party feature flag system, that is almost always going to be more valuable than building it in house. It gets me back to mainline work.
I agree mostly but I would say it's a bit more nuanced. Instead of what's unique to my product, maybe also consider what's unique to my market. Business is about selling stuff. If you are selling to a unique subset, all you need is the same feature set but which feels different.
If I can take X to drop in a 3rd party feature flag system, that is almost always going to be more valuable than building it in house. It gets me back to mainline work.