I'm suggesting it's essential to use third party services to do the things that aren't innovative, so you have the time, runway, and focus to do the innovative things.
For example, no SaaS startup should be writing their own payment system unless they're a fintech startup doing payments. If you're wasting time building things that you buy in for much lower cost then you will fail.
I'm not saying not to use third parties, I'm disagreeing with using the term essential in that context. It is too encompassing and final of a statement. To me it implies that any innovation in the space cannot possibly happen without the usage of third-party services, which is simply not true.
For example, no SaaS startup should be writing their own payment system unless they're a fintech startup doing payments. If you're wasting time building things that you buy in for much lower cost then you will fail.