Steve Blank has an interesting definition for a Startup "a startup is a temporary organization used to search for a repeatable and scalable business model."

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.

They’re a long-established company with a mature product.

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.

Or why are they still unprofitable if they have 10s of thousands of paying business customers. Are they planning on becoming profitable when they have 100s of thousands of paying business customers? Is it possible that its very hard to keep scaling after a certain threshold to meet the needs of all these business customers that require different kind of features.

For what it's worth WP Engine didn't raise "hundreds of millions of dollars." Their most recent round was a PE deal that resulted in the investor taking a controlling interest in the company. Most of the $250 million figure went to buying out option holders.

I worked there for 2.5 years (a great experience) and it mostly felt like a mature business rather than a growth stage startup.

I worked there too back when it actually was a growth stage startup. What you say only further proves the point: I don't understand how it is a "startup" when there is no exit left for founders and employees. Their exit pretty much happened already.