I don't think most foreigners who have amassed so much wealth outside of America that they could throw $5M at a US citizenship path would care to do so. First of all, you've already made it. Once you have that kind of money, you can live like a king in most countries around the world. Not to mention, you are likely older in life and have deep roots elsewhere.

But most importantly, with that kind of money, why subject yourself to a lifetime of US taxes? Instead, just visit whenever you'd like to as you'd easily be granted visitor status as needed and enjoy the normal residency based taxation that almost every other country in the world adheres to.