The fact one person gets negative bills doesn't mean it's unsustainable. It's only unsustainable if it costs more in energy storage + standby infrastructure + your negative bill than it costs to run the usual energy production pipeline.
That may be the case, but it's unrelated to whether you're paid back or not.
It's unsustainable in that many people jumped on this deal, far more than forecast, and so they pulled it years ago. The government had essentially subsidised solar through feed-in tariff bonuses, but hadn't expected the volume of people who ultimately took it up.
It's also my understanding is that's not how the infrastructure works. It's simply not set up such that my feeding in could be sent arbitrarily to a storage point. Indeed, a colleague of mine was having his inverter trip during summer because his + many of his neighbours were all feeding in however none were drawing out. The grid was therefore being overloaded locally and all of the inverters were hitting their automatic cut-off points because of it.
So yes, in theory it could be sustainable, but this particular situation absolutely wasn't and if you were to get solar now, you would not end up making money off feeding in like I do. It's now purely to offset self-usage.
That may be the case, but it's unrelated to whether you're paid back or not.