If you can't make your business work on $x * 70%, you can't make your business work.
Every SaaS faces the same issue you have. And they (mostly) still offer IAP. That's because you might lose the 30% on the first sale, but usually renewals happen out of band.
What do you mean that we will lose 30% from only the first sale? If a user subscribes using IAP, their subsequent payments will also be charged a 30% fee by Apple. Won't they?
And its not the question of whether we can do business at $x * 70% or not. We would love to give 30% to someone who actually brings a customer to us (we actually have a 20% recurring commission for our affiliates). The question is how is Apple entitled to that cut?
Every SaaS faces the same issue you have. And they (mostly) still offer IAP. That's because you might lose the 30% on the first sale, but usually renewals happen out of band.