No matter the manufacturer, every Android phone has one thing in common: its software base. Manufacturers can heavily customize the look and feel of the Android OS they ship on their Android devices, but under the hood, the core system functionality is derived from the same open-source foundation: the Android Open Source Project. After over 16 years, Google is making big changes to how it develops the open source version of Android in an effort to streamline its development.
Degoogled versions are absolutely not popular and definitely not the cause for this. In fact for all that matters nothing really changes - they’re still releasing the source code for AOSP.
Every good part of Pixel OS, OneUI, ColorOS etc is already closed source. AOSP is just the outline of the bones they’re built upon.