Is it not that once on the Path, one needs no external help until one reaches Arahanthood?
That is, within seven bhava the Arahanthood will be attained (if it is not implied that those bhava take place in a Buddha sasana AND in a family with access to the Dhamma, but THAT would then be ‘external help’). But a bhava in a Buddhas sasana is extremely rare.
I see it like this:
If ‘one would just be a Sotapanna for the duration of that life’, and that may happen over and over for the next six bhava, then no progress can be made and Arahanthood will not be attained, for by far the most bhavas are outside of Buddha sasanas,or in families where Dhamma is unknown, yet they add up to the necessary seven, so progress will be made even outside of a Buddha sasana,or outside of a Dhamma environment in a Buddha sasana, given that no further external help is necessary.