There isn't anything stopping individuals from publishing Safari content blockers, though? No more than with the previous extension model.

Nor were individuals the only players to take advantage of the powers available to old extensions.

There are also legitimate security reasons why browsers such as Firefox have restricted side-loading add-ons on consumer operating systems such as Windows.

There's a debate to be had about whether security for most users is worth the trade-off in flexibility for power users. But regardless of any other reasons we might speculate, the arguments that have been presented in favor of such changes are already compelling enough on their own and need to be seriously addressed.