i think since the phrase has a proverbial nature it would be nice to put it in the gnomic aorist, a thing latin and greek were most fond of, though rare in english (cf. perh. "curiosity killed the cat").
mors and vita are pretty ineluctable words to use, though i think succedo may be the verb that most aptly captures the changed state from one to another.
for the sake of euphony: vitam successit mors.
i wouldn't have thought of using mortalitas, although it is nicely reminiscent of tacitus or perhaps the quotation from cicero's de natura deorum: ...aut non omne quod ortum sit mortalitas consequatur.