If the man who is unclean does not cleanse himself, that person shall be cut off from the midst of the assembly, since he has defiled the sanctuary of the LORD. Because the water for impurity has not been thrown on him, he is unclean.
God gave ancient Israel certain practices regarding ritual impurity in His worship that do not correlate well to the New Testament experience of life in Christ. This chapter is one of those kinds of things. At issue is not whether the ashes of a dead bovine mixed with water essentially convey holiness. The point is obedience to God’s command on the subject. Certain life experiences rendered a person “unclean” regarding contact with God in worship or contact with His people in the assembly.
The “water for impurity” made from the ashes of the red heifer was used daily no doubt. One of the main situations described in this passage involved ritual impurity when coming in contact with a dead body. And people lived and died in the desert camp daily. So on any given day there were hundreds of people who had become ceremonially unclean. The practices in Numbers 19 brought them back to a ritual purity that allowed full participation in worship again after confronting the death process personally.
We tend to read hygiene into this sort of thing. That was not the issue. I think the reason ritual impurity existed at the presence of death was to remind Israel that the curse on sin is death. There is a consequence. Paul called death the wages of sin for good reason. Death separates us from God because sin separates us from Him. Participating in a ritual that restored purity emphasized this over and over again in Israel.