Until stream-entry, convention and identity drive for continued existence guided by craving. That unrestrained craving results in endless rebirth.
With stream-entry, the Noble Truths are understood as true, and the perspective on identity view changes. Identity view is now something to relinquish rather than develop.
MN64:6.1: But an educated noble disciple has seen the noble ones, and is skilled and trained in the teaching of the noble ones. They’ve seen good persons, and are skilled and trained in the teaching of the good persons. Their heart is not overcome and mired in identity view, and they truly understand the escape from identity view that has arisen.
In other words, “build upon one’s progress” doesn’t quite characterize stream-entry. Instead, there is increased contentment with the beautiful sufficiency of “enough”. Indeed, here we have Ghaṭikāra from MN81, a non-returner, well beyond stream entry:
MN81:18.13: He takes what has crumbled off by a riverbank or been dug up by mice, and brings it back in a carrier. When he has made a pot, he says, “Anyone may leave bagged sesame, mung beans, or chickpeas here and take what they wish.”
The story told that a bodhisatta nobly delays their enlightenment for the sake of others feels a bit awkward to me personally in that it smacks a bit of martyrdom and self-mortification. Looking again at MN81, we see that Ghaṭikāra does not go forth. Instead, he rather enthusiastically and firmly latches onto the student Jotipāla’s hair and insists that Jotipāla see Kassapa Buddha. In this Ghaṭikāra is behaving somewhat like a “bodhisattva”. But his reason for doing so isn’t to accumulate an all-time high score of Buddhist converts. Ghaṭikāra’s reason for not going forth is humble and simple.
MN81:11.3: ‘Don’t you know, dear Jotipāla, that I look after my blind old parents?’
That was Ghaṭikāra’s last life. He did not return. Jotipāla went on to become Gautama Buddha.
Regarding the number seven, I wouldn’t get caught up in specific numbers. For me it sufficed to see that in the day-to-day nobody asks for seven more desserts when one or two are enough. The opportunity to practice the Path as a human is very rare. Many, oh so many, are born in circumstances not conducive to the practice. And if we are truly appreciative of the gift of this very human life, why would one be so greedy as to want seven more? Perhaps one might instead be somewhat embarrassed at monopolizing the gift of a body that offered the opportunity for another to use on their own journey forward? How many lives is enough? For Ghaṭikāra, one was.