I have been using Supermemo daily for almost 20 years, and I have basically stopped trying to sell people on the idea. When I explain it to them, they agree that it is a good idea, but very a few people can truly follow through in the long-term. Yes, SRS is basically a gym membership that most people will stop using after Jan 3, I like that metaphor.
I’ve been using a spaced repetition flashcard program called SuperMemo every day for more than 15 years. It has worked really well for long term retention on stuff I’ve made flashcards about, which includes languages and other stuff I find interesting.