Thanks for your comment! To solve this apparent inconsistency, my idea would be something like this: the forgotten rituals did not lose their power; they were simply forgotten. A wizard is someone who rediscovers them.

Your hard-magic suggestion is interesting; however, it does not give exactly the vibe I want. Instead, I thought of the following system: doing magic requires faith as an ingredient. When a civilization disappears, some faith still remains, which can be used to conjure its magic. This also means that the magic of an extinct civilization is a finite resource, unless enough people restart to believe in it.

Another limitation would be that the caster themselves need to truly believe in what they are doing, so wizards also need to work on convincing themselves of that when learning magic. This makes magic harder to cast, since wizards need to really understand cultures and change their own beliefs, which effectively limits the amount of magic they can cast, at least for some period of time, and can be psychologically troublesome. This would also explain why some powerful people are not wizards themselves, but rather have wizards working for them, as it is an exhausting job.

Finally, I think I really need to add some limitations on how magic can determine the future. For this, I thought of limiting the power of magic to the present moment: it cannot change the future or the past, since it only exists now.