If you had separate blockchains for each currency instead of one, wouldn’t you be effectively splitting Nu into 4 separate businesses? While the shareholders would be the same in the beginning, ownership between them would grow apart over time. This means you would need to pass the same motion on 4 separate blockchains, which may not be possible due to differences of opinion between shareholders.
With 4 groups of NuShares supporting one currency each, Nu would inevitably fracture into separate businesses as voting for each currency would no longer be tied together through the ownership of a singular group of NuShares. NuShare value would now also be split 4 ways and you’d run into issues, for example with shareholders of EU-NBT or others not wanting to work together to help back the value of US-NBT if it got into trouble again.
You’d also need separate branding and logos such as US-NSR and EU-NSR as you could no longer keep calling them simply NSR if there were 4 different types.