I feel like you've proven the point in the opposite direction. A software startup may be competing with Salesforce as an alternative. That would be worth pursuing as a software startup. You've laid the case that it's better and easier to create internal tools and not rely on Salesforce or an alternative solution. It's easy to roll-your-own so you don't pay outside companies for this solution any more. This does not bode well for a software startup (maybe it bodes well for you as an engineer working on that project, but that's not a software startup).

I feel like most "solutions" that are really just "platforms" and require extensive customization and integration will suffer due to this. It's easy to just roll your own these days.