Your are right except an exception Shopify might be used in some very small parts of big brands with few SKU's. For moderately large SKU most use flexible systems and shopify or BigCommerce is not one of them. Indeed like many companies even if one small division in one country of an MNC or brand is using BigCommerce or Shopify they will add their logo on their site to show they have this big brand as their customer.

Having worked for many multi-national brand can say they use Magento, Hybris, Demandware (now salesforce commerce) more than Shopify or BigCommerce.

Indeed even all those brands can be served better by open source multi-channel commerce system whose source code they control, otherwise they might have same issue when Oracle and IBM stop investing in ATG and WebSphere commerce platform. Indeed many of our old customer moved to in-house system as ATG, WebSphere commerce and even hybris cannot keep pace with the fast paced changes required in multi-channel commerce. Shopify and BigCommerce will need to primarily serve their SaaS customers and being limited by single code-base for all customers, cannot serve the needs of individual brands or multi-nationals that effectively. Even if they come with a program to provide on-premise or deployment option on a cloud of customers choice, given domain expertise they still will not be able to provide that flexibility offered by customized solution.

So for those brands its a better choice if they retain control of their core multi-channel commerce platform than leave to the whims of Shopify or BigCommerce which are anyways not that flexible and all the additional functionality needs to be developed in-house with partner using plugins or extensions.