(Bamidbar 19:20) "And a man, if he becomes unclean and does not purify himself": Scripture speaks of defilement of the sanctuary and its holy things, and its punishment is kareth (cutting-off). But perhaps the kareth is for (not) sprinkling? It is, therefore, written (Ibid. 12) "and if he does not purify himself (with it) on the third day and on the seventh day, he shall not be clean." His punishment (for not purifying himself) is that he shall not be clean, and not kareth.
(Bamidbar 19:21) "And it (the sprinkling of the waters) shall be for them for an everlasting statute": that it be observed throughout the generations.