For those 100+ there are many companies and service provider which will help them built a highly customized and resilient system based on these open source that too with better flexibility and still very cost effective. The advantage offered by Shopify and Bigcommerce is good for companies with few SKU's or mom-pop kind of stores. If you really go through the links you can see the quality of the code and ease of use, its much better than what's offered by both Shopify and BigCommerce.
Also as the source is under the company using the system, they maintain a full control over their own destiny instead of depending on the whims of Shopify and BigCommerce.
Also as the source is under the company using the system, they maintain a full control over their own destiny instead of depending on the whims of Shopify and BigCommerce.