This is like the old saying - statisticians will happily tell you how long your nails will be 40 years from now (with the mild assumption that you won't cut them).

A country of 100M people will change its birth patterns long long long before it "disappears".

There are probably subgroups of Japanese people with birth rates higher than replacement. There may also be major advantages to having a child in 2100 that are not evident now (vast tracts of unclaimed land to exploit?).

The only way populations go to zero is in a Children of Men situation, if the city dwelling populations all choose not to reproduce eventually the country will have a lower population by be more comprised of religious cults with high reproduction rates