I see all sides. We’re adults & can be civil here.
If ‘Undo’ were an option that could be turned on, perhaps they could add a humorous warning ‘Beware! The Crabigator will be very disappointed in you if you use it too much.’ & a funny gif / link to a gif.
If you really want to cheat & just advance thru levels fast without actually learning, well that’s the user’s choice. Stupid choice in my opinion, but do what you want. I’m (& I assume everyone) here to learn. Rikaikun, dictionaries, other tools aren’t allowed when I do reviews. Use / don’t use what you like.
I used Undo as a tool to help me learn. Without an Undo button I tense up, slow down when doing reviews, sometimes check my notebook before entering an answer. Frustrating, discouraging when I make a typo - I know the answer - ugh I typed ぎ not ぐ. Mistyping o i u is a common mistake for me in daily life. With an Undo button I like to think fast, enter my answer. “Ya know it or you don’t.” In daily life there’s no time to ponder every kanji. If I hit ‘undo’ I ask myself ‘do I actually know it, or was it a stupid mistake?’ If I guessed something right but I don’t feel I know it well enough I intentionally enter a wrong answer so it comes up faster in the queue. With Undo, I think I marked some correct answers as incorrect for that reason.
For me, reviews are a learning experience. Daily life & JLPT are my tests. I have a huge poster in my classroom “Make mistakes. It’s ok. Learn from your mistakes.” At the start of a term, some students are hesitant to speak, afraid to make mistakes. In class they can relax; it isn’t a test. Learn from your quizzes, other mistakes. Midterms, final exams, TOEIC are the tests. Love seeing how the students improve. Want to get good at a skill? Practice, practice, practice. Draw lots, cook lots, keep working to the desired result. Practice your musical instrument or sport a lot & keep making improvements. Enjoy your practices, keep making progress. If you’re scared to make mistakes you’ll practice less, won’t improve as fast.
Some WK users don’t care at all about Undo. For me, oh gosh I miss it, enjoy WK less, reviews are an unpleasant chore. With Undo I wasn’t scared to make mistakes. When speaking with Japanese friends I’m not worried about mistakes, feel free to speak more.
Be nice, everyone.