Not a SalesForce dev, but there is a bit of oversimplification what CRM SaaS is today.

Almost everything integrates with SF today and most often understanding, replicating and maintaining these integration pathways may need more than 1.5 engineers. You then bring 3 engineers (to cover absences) and buy enough tokens.

And we haven't even scratched other parts: disaster recovery, security, legal (CCPA/GDPR), etc

I would say that the main reason to buy corporate software is to delegate responsibility to a 3rd party. By doing in-house you take the legal, regulatory responsibility of pretty sensitive records, that will definitely bite you in the future, or will easily inflate the build costs