Do you think Android will one day become so restrictive that sideloading (installing apps outside of Playstore) will become extremely limited, or completely banned?
If they are going to restrict sideloading, they'll probably restrict bootloader unlocking too. In the future, just make sure you research the bootloader unlockablility before buying a phone, because the next pixel might be locked.
It's cellular providers who restrict bootloader locking generally. But you're right but that would go against the grain as the Pixel series are designed to be the flagship for AOSP
AOSP at the very least, but the average consumer doesn't really care much about Google Play Services or their Mainland China substitutes (obv cos great firewall).