After-the-fact analysis of the causes of major disasters often reveals multiple independent causes, none of which would have caused a disaster by itself, but each of which degraded or disabled the usual safeguards in place for the other problems. This seems to come up in everything from relatively small-scale transportation disasters to the fall of civilizations, and possibly in major extinction events. E.g. there have been many large asteroid impacts, but the one which finished off the dinosaurs happened to also coincide with (and possibly triggered or exacerbated) major volcanic activity. (The Deccan Traps.)
So the worst possible outcome of the epidemic might be that it happens to coincide with some other, totally unrelated disaster. For example, natural disasters such as earthquake+tsunamis, widespread rainfall and flooding, major fires piling air-quality issues on top of COVID-19 breathing problems, and so on. (In a way, I'm thankful the recent fires in Australia happened then, and are therefore not happening now.) Unrelated war(s) would make everything worse. So would a second pandemic at the same time. So would just about anything on the list of possible existential risks. I think this would count as a worst-case outcome of the epidemic, even though it would be an indirect outcome.
The global scale of this epidemic, and its months-long projected duration, seem to make it more probable that something else will go badly wrong just when everything else is under stress.
Also worth noting: if the onset of global catastrophes is better, then global catastrophes will tend to cluster together, so we might expect another global catastrophe before this one is over. (See the "clustering illusion.")
I once took a look into the clustering illusion, and found a research that in the interconnected systems it is not an illusion: any correlation increases the probability of clustering significantly:
Downarowicz, T., & Lacroix, Y. (2011). The law of series. Ergodic Theory and Dynamical Systems, 31(2), 351–367.
Generally agree, but I would think that some kind of "new waves" scenario has a much higher than 1% probability given that it's how influenza and many other seasonal viruses continue to circulate. That said, in many "new waves" scenarios we are able to create seasonal vaccines or otherwise mitigate the effects. This may result in a permanent drag on economic growth without reducing growth below 0 (collapse).
Once enough people have been infected and recovered, gaining immunity, the evolutionary pressures on a virus switch from "spread as fast as possible into new hosts" to "keep the current host alive and infectious long enough to encounter a host without immunity". Even though influenza periodically bypasses existing immunity, the evolutionary pressure towards lower mortality is still present most of the time. In particular, our actions to quarantine and isolate, if sufficiently widespread, will also put a lot of evolutionary pressure towards less-severe effects on SARS-CoV-2. All those mild and asymptomatic cases? Pretty soon those are going to be the most successful replication strategy, and the SARS-CoV-2 population as a whole will be pushed towards causing lower mortality.
There is still a chance we'll have repeated high-mortality waves, but one should note that coronaviruses and influenza viruses are not particularly closely related. Influenza seems to have about one mutation in its protein coat per replication, while as of Feb. 11th the 81 sequenced samples of SARS-CoV-2 had "at most seven mutations relative to a common ancestor". So I'm inferring that SARS-CoV-2 is less likely to be able to bypass existing immunity on a yearly basis. Influenza has a high mutation rate due to lacking RNA proofreading enzymes, so if SARS-CoV-2 has an RNA proofreading enzyme or hijacks host-cell proofreading enzymes I would also update towards a lower probability of repeated waves. Influenza is also unusual because it's composed of eight pieces of RNA, which makes it easy for different strains of influenza to swap genes when they infect the same cell at the same time. This is another major reason that influenza bypasses immunity so often. SARS-CoV-2 seems to have one 30,000 base-pair segment of RNA, so it can't do that trick either.
There are still a lot of unknowns, but so far there's no evidence I've heard of which has made me update towards SARS-CoV-2 being more likely to be able to bypass existing immunity than other coronaviruses, much less influenza.
Coronaviruses in general, including the original SARS-CoV, do indeed have proofreading enzymes, which is why their genomes are so unusually stable (and hence so unusually large) for RNA viruses.
I didn't dig down on the SARS-CoV-2 genome yet (I only started looking into this in depth yesterday) but a quick search suggests that the proofreading exonuclease is indeed there.
There are several possible ways how current pandemic may turn into a civilizational collapse (which may be the first step of an extinction event, depending on the possibility of recovery). I will list below several putative ideas of how the pandemic could turn into a global collapse. Each of them, I guess, has lower than 1 per cent probability: