why plan it? who knows if he will be alive to abdicate by then, just do it now. most times people plan to step down from some job or position they already have in their mind and are just phoning it in for the remainder of the time.
Likely to give the nation time to prepare for the transition and to plan for the pomp and circumstance (public celebration).
The Japanese have the highest life expectancy of any industrialized nation.
IIRC, when a Japanese Emperor dies/abdicates, the calendar is reset. Lots of preparations to make.