Keep in mind that this is in the context of the autodidactic student. Not everyone is lucky enough to be able to study mathematics full time. It must be nice to make use of your maths knowledge so frequently that you don't have time to forget anything, but it is not the reality for me.
In my experience with language learning, Anki does not feel like a chore and is not taxing at all. It is actually fun and takes less than 5 minutes a day. It will take longer if you have tons of cards to review, but it is pretty easy to manage the workload so that it never happens.