It is possible that our website is 100% WCAG compliant and still not accessible to an assistive technology user. WCAG alone is not enough, we must test manually as well.
Scaling down web browser is not enough. We should really test with physical devices and with at least Android and iOS based devices. UI and UX tests are important but so is testing accessibility with mobile screen-readers.
Why are we not encouraged to use ARIA everywhere? Because it is a last option – if we are not able to find a native semantic element, or if we want to create something special, then we can use ARIA. But it should be used wisely!