You just can't reliably work on one artifact with dozens of people without creating a lot of complexity, this is why microservice architectures can be better in some cases.
If you're not going to increase the development team beyond a single smallish group of people, you'll be much more productive and your service will be way faster as a monolith.
I'm pretty sure most SaaS companies want to scale their development teams, so it's basically a given that most want to use microservices as well.