This is quite reductive of the situation. Android is not going anywhere, and neither are the third-party app repositories.

AOSP will be further restricted, but that really only affects anyone who doesn't use LineageOS / GrapheneOS / your favorite ROM. The Venn diagram people who care about side-loading, people who can't afford to switch to a supported device, and whose device is new enough to support the side-loading restriction is not zero, but they will be smart enough to go through the process of enabling side-loading.