These organizations, while revenue generating, may still have some way to go before they can reach a stage where their revenues and costs are predictable and the business model sustainable.
For those interested in the history, BigCommerce is the SaaS delivery of Interspire. You can’t buy Interspire directly anymore, I think. But I did have the displeasure of customising an Interspire source-code installation many years ago and finding it to be clunky and poorly structured PHP. I haven’t seen under the bonnet of Shopify but I found it much easier to build and integrate for. They talk about their architecture and engineering often at Rails conferences, and I find this a deal more confidence inspiring.
I worked there for 2.5 years (a great experience) and it mostly felt like a mature business rather than a growth stage startup.