The point is, compared to Magento 1, where a pseudo fallback mechanism was implemented. Magento 2 has a mechanism that falls back as many times as a parent theme can be found. The system is very similar to the one introduced on Magento 1.9 using
For instance, when you have 5 themes inheriting from each other, then the mechanism will fall back to each one of them. The package is not taken into consideration anymore.
Here is a pseudo code that can describe the new design mechanisme:
look in the current theme
current theme will be parent theme on the next iteration
while the current theme has a parent theme
look in the view folder of the module area if the layout/template is not defined in the previous steps
As an example of static file fallback, suppose there is a request for the
logo.svg image in the Blank theme.
Using theme inheritance and view file fallback rules, Magento uses the following search order: