I don't see retrospectives being that much of a time-sink, it's a 30min/1hr meeting every 2 weeks. Even when I was on a kanban team they still had a fortnightly retrospective, it's useful to have a checkup occasionally.
Every time someone hates Scrum they always complain about 2 week sprints, which are an insane choice (should be 4 weeks), that shows they aren't doing Scrum, they are micromanaging with "Scrum" as the excuse.
Yes, I don't know where the 2 week sprint thing came from, but everyone uses it as a default. I've tried to get it to 3 or 4 weeks but always get resistance. The main reason I think is the business wants their features NOW.
Going longer than two weeks for a sprint just makes the lies happen at different place. There are approximately 0 teams that can accurately state all the work they will need to do four weeks in advance, even two is a stretch. Planning and retro per sprint become a burden though, on less than two week sprints.
I currently work at a place that does 1 week sprints. It's bad.