There is a bug or weird behavior with {% sw_extends%} and with {% sw_include%}
Try {% extends %} {% include %}`
https://github.com/shopware/platform/issues/614
TLDR:
Currently the sw_extends looks for the next inheritance chain entry which is queued after your „MyTheme“. However, this algorithm does not seem to consider if the extends is not the same template as the loaded file.
So at the moment it is right that you have to work with extends. I have imported the ticket and we will take a closer look at the error.
Just to make sure use the Symfony profiler to see which file is been loaded
If your file is loading from @Storefront, that’s the issue