Wordpress is the Craigslist of Website Builders
The fact that software exists out there as a virtually free way for anyone to build websites is both an issue and an opportunity. Note that when I say "free" I don't mean in a total cost of ownership way, but I mean it's a well-supported, and reasonably-priced way to do it. Not forever perhaps, but a way to get started. You don't need to look futher than the fact that Wordpress is 43% of all websites, and 60+% of the CMS market according to the Google.
This reminds me of Craiglist. Of course, Craigslist was disrupted by eBay, Airbnb, Amazon, Facebook Marketplace, LinkedIn, and hundreds of other small niches ... yet Craigslist itself has a large niche that is still going strong. Similar for Wordpress.
The technology behind Wordpress runs 162 times more websites than Shopify does. How many niches of websites is that? Quite a lot of niches. And many are not enabled for eCommerce, or if they are it's very basic and entire industries are only "Day 1".
There are 30+ million eCommerce enabled websites worldwide, which is 6x the Shopify number of ~5 million. Which is about the same number as I have seen for WooCommerce.
What does that mean for other providers? The delta between 30 million total eCommerce stores and the 10 million provided by the "big 2" providers is quite wide, and that is assuming most industries are digitized already (aka zero sum).
Of course that is ludicrous. How many doctors or lawyers or dentist offices have you walked into that were using green screens? Quite a lot.
Let's speak about a popular topic here - the future of BigCommerce. I think it can be very bright. And there will be plenty of places for many winners across all eCommerce. It just may not all be in the same segment.
Let's assume that BigCommerce powers less than 100k stores (the numbers I have seen are ~60k). This means to triple its website growth, BigCommerce has plenty of available blue ocean to tackle.
For any eCommerce professional, I would encourage you to look beyond the zero sum game of "direct to consumer fashion retail" because it seems like the hot thing. It's also the bloodiest red ocean that exists.
Want to find that next hundred-million dollar niche? I would encourage you to look to Wordpress for inspiration. All the answers are already there.