Hey there,
I share the frustration, and I believe this hasn't hit Shopify's roadmap due to their target audience for the platform - which are primarily vendors looking to establish an online storefront, or entrepreneurs looking to dip their toes into E-Commerce.
With that said, I've been working with Shopify for a number of years, and I've recently done some writing on how to establish a Theme Inheritance/Boilerplate/Parent-Child Structure for theming in Shopify. The solution doesn't lie in Shopify, but rather a DevOps approach to making this possible with some Git magic!
I am sure this can benefits many Shopify Theme Developers out there that are focused on DevOps and want to establish a modular approach to theme development in Shopify.
You can find that article here: https://safrazssears.medium.com/creating-a-theme-inheritance-boilerplate-structure-for-shopify-theme...
I hope it helps!
