At some point you start getting lots of customers who purchase less but are more likely to require significant customer support resources for what they do purchase. At scale, things that might have looked O(n) at one volume of customers can change.

Even if you could double your customer support resources, it means a very different structure and needing to ensure you both hire in the same quality of employees up and down the escalation chain as well as adapt your organization to handle the vastly larger communication and problem resolution processes.

Noe of that is trivial and it's quite easy to take a great org and make it terrible by just adding a bunch of people.