You can do 99.5% of what most non-technology businesses do via a combo of SaaS services. Those keep getting better over time. It’s cheaper to hire a large team of coders/consultants for a few months to migrate you than to pay the same said team in perpetuity.
What is is with tech people and pulling numbers out of thin air to justify their simplified straw man of users?
I agree that pretty much any large organization writing their own email or payroll system in this day and age is probably certifiably insane.
But even large "non-tech" companies have fairly significant custom needs even if it's just integrations and customizations of SaaS. Just because you're using Salesforce doesn't mean you just point everyone in the company at your Salesforce instance and call it a day.
What you're forgetting is that insurance companies still need their custom insurance software with all of their custom risk models built in. That's the entire value proposition of the business so your estimate that it's only "0.5%" of what the company needs is way off.