Roxana Cooper Wrote:
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> Enough boats were lost to convince the remainder
> there were no more worlds left
> to conquer??

No, that wasn't the mechanism by which they determined 'there were no worlds left to conquer'. It certainly wouldn't explain why the hull design changed throughout the Pacific.
They knew because there was ongoing two way voyaging after the initial colonizations.
How do we know that, one might ask? Because of DNA studies such as those done by Rebecca Cann (of the University of Hawaii) which sought to explain contradictory genetic support of both the 'Express Train' model and the 'Tangled Bank' hypothesis of Pacific settlement patterns.
"By examining different genetic systems from the same individuals,we have generated patterns consistent with both views. As described above, our mtDNA data are correlated with linguistic data and suggest Island Southeast Asia as the origin of Remote Oceanic Islanders. These are data consistent with the express train model. Our STR data, in contrast, are not correlated with the linguistic data and highlight the affinities between Near Oceanic and Remote Oceanic populations. We have argued that the differences between these two patterns result from post colonization male-biased gene flow. Genetic interactions between populations after initial colonization may have been mediated by a predominately male segment of voyaging societies, engaged in the control of resources. This bias served to preserve pre-existing linguistic differences, lines of status, and hierarchal differences among matrilineal kinship groups. Thus we see female settlement as an express train, and male gene flow as a tangled bank."
Lum, Cann, Martinson and Jorde Am. J. Hum. Gen. 63;613-624 1998