Are SaaS businesses really customizing for high paying customers? They may build new features which are then made available to other businesses but I would be surprised if there are separate source trees.

The other issue is that a "great business" and "scale" are not synonymous. In fact many of the current batch of companies of great scale, like Slack, have spent enormous amounts of money to get there, and still lose money after becoming public. Then you have a company like Atlassian, with a confounding mix of enterprise products that are hard to use: but a fantastic businesses that's been profitable since the very beginning.

I do agree with you that the APIs, webhooks, and Zapier integrations solve the problem of customization, although I'd still say it's less customization and more a bevy of options.