Counterintuitively, ultra-low Earth Orbits are not about less sustainable, but rather about more sustainable and eco-friendly spaceflight. If done, right: which would mean real air-breathing thrusters, which could (theoretically) allow satellites to stay in low orbit indefinitely, without needing more propellant.
The problem with New Orbit startup is that while they declare this intention, their actual Aura thruster is xenon powered, so the demonstration mission won't actually be sustainable (as I understand it).
The low orbit has numerous advantages - eg. reducing impact on astronomy, because low flying satellites would pass overhead very quickly and most of the time would not be illuminated by sun. There is also zero risk of Kessler syndrome, because most satellites would re-enter quickly. But prolonged lifetime of such satellites is essential, to reduce dangerously high number of launches and re-entries.
https://neworbit.space/news/can-a-humble-startup-boldly-go-where-no-spacetech-has-gone-before