These are fair points; I guess the feasibility of spaced repetition comes down to what you consider a repetition. If you're considering a single quick problem on a topic to be a repetition, and you don't have too many topics, and you're not plowing through them too quickly, then I can see it being feasible as you're saying.
Math Academy is built with a different context in mind:
-- 1. we have tons of different topics (for instance, over 300 in our AP Calculus BC course -- and that's just one course; students who stick with us past their first course and continue taking more courses on our system can easily accumulate thousands of topics on which they need to maintain their knowledge)
-- 2. each repetition consists of multiple questions on a topic (after an initial lesson task consisting of somewhere in the ballpark of 10 questions, future repetitions are review tasks in the range of 3-5 review questions)
-- 3. students are getting through a new topic every 20 minutes or so on average (our AP Calculus BC course is estimated to take about 6000 minutes, and that includes time spent on review / quizzes / etc., and 6000 minutes / 300 topics = 20 minutes/topic)
Just some back-of-the envelope math in our context: say you learn 3 new topics per day (an hour-long session), and each review takes you about 4 questions. Then, as a rough estimate, pretty quickly you reach a point where you've got
12 review questions based on yesterday's lessons,
+ another 12 review questions on topics you reviewed for the first time last week,
+ another 12 review questions on topics you reviewed for the second time a couple weeks ago,
+ another 12 review questions on topics you reviewed for the third time a month ago,
+ another 12 review questions on topics you reviewed for the fourth time a couple months ago,
...
That's already 60 review questions/day, already past the point where you’re spending every hour-long session entirely on review, which means that your progress grinds to a halt in terms of learning new material.
So, in our context at least, raw spaced repetition just doesn't work out in a way that’s feasible – student will get absolutely crushed by a tsunami of review unless we unless we take measures to drastically cut down the amount of review (i.e., fractional implicit repetition + repetition compression).
I hear your point about needing some refresher if you take 2-3 months off and come back, though. Similar thing happens with Math Academy students if they take time off and come back, but the way we solve that problem is we just recommend they take a new diagnostic to refresh their knowledge profile. Basically, it just peels back their "knowledge frontier" to a point where they can pick up and continue learning smoothly with our standard approach.
(In my experience, that’s what you’d ideally do as a teacher after summer vacation – you know your students have forgotten a lot of material, so you have them take a beginning-of-the-year knowledge evaluation and go from there.)