New web APIs, for sure. But the screen reader market is not fast moving, in terms of new software being adopted. The line-up of the most used three screen readers (NVDA, JAWS and VoiceOver) has not changed in over a decade, despite the individual software applications themselves undergoing changes, and of course the market share of each one increasing and decreasing over time.
> Do we seriously expect every small non-technical business eking out a living with a small store to be experts on every facet of accessibility?
No, but I also don't expect such a business to be up on the latest in security, PCI compliance, GDPR conformance and more. For that reason, they are probably either:
1. engaging a web design/development agency; and/or 2. using a pre-defined platform, like Shopify.
In the former case, I do expect anyone making money from website building to at least give accessibility some thought. For the latter, Shopify is one of the businesses you describe, as a "large tech company who can write a blank check for a large team of full-time developers who can work full time on nothing but accessibility". As such, they absolutely should be setting up small business owners for success, by making their out-of-the-box themes, widgets, flows, etc. reasonably accessible to the widest possible audience.