The good news is that that unsold carbon won't be burned, and won't be reintroduced to the system in future auction allocations. Its gone for good. Which, insofar as ETS allocations reflect our desired emissions pathway and/or actual future emissions, is a good thing. But its not how the ETS is meant to function, and an artificially low carbon price set by political expectations is not going to provide a financial incentive for polluters to decarbonise. Instead, it does the opposite: encourage them to keep on polluting. Which is not a good thing at all.
On the other hand, if its all about political expectations now, maybe the opposition parties - who if we remain a democracy will be in power some day (and hopefully sooner rather than later) - should just start setting them directly. They've already signalled that the gas industry has no future, by announcing the intention to restore the drilling ban. Maybe they should start setting similarly direct expectations for coal users, or for fossil cars, or for our biggest polluter agriculture? Or would that be too great a departure from capitalist orthodoxy for the "Labour" party?