Circumcision
When something from nature/fitrat/daily life is taken and raised to the level of religion by the Prophet, then its status as religion takes precedence over the rationale behind that status. Of course, rationale does help us in understanding the overall framework of the religion but will always be subservient to its religious status. For example, if I look at the rationale of eating with right hand (which is a religious act), I know that it’s purpose is to keep myself reminding of the day of judgement with every meal that I take. Now that I can use so many other means to keep myself reminding of that (e.g. I can set a reminder on my phone for that or I think 5 times prayer is enough to remind me of that), we both know this will not allowed as a substitute for eating with right hand. Similarly, in my opinion, though rationale behind circumcision is purity/tazkiya, but at the same time, this act itself was made a part of religion, so it can’t substituted with other cleanliness measures.