I don't think this is a foregone conclusion by any means. Many countries have gotten growth rates low enough that even after 2 years, their whole population won't have gotten it. Some countries might opt to infect everyone (as the US seems to be doing, due largely to chaos and incompetence), but I don't think it's the only option.
Source? It depends on the intervention, but my understanding is that that is not required to be true on the 18mo timeline. I.e. I think the original Imperial College study was calling for for suppression, rather than just avoiding overwhelmed hospitals
There are a lot of useful interventions that we're figuring out that will help even before we get a vaccine. Lots of antivirals are being studied and those that work will probably be best given in the first few days after symptoms show up, which requires more testing. Figuring out when to give steroids or IL-6 inhibitors after the first week when the virus starts to fade but the danger of cytokine storms gets worse. Figuring out when to flip people onto their stomachs (proning) and how low blood oxygen levels can get before intubation is actually necessary.
If we can keep this thing suppressed for another month summer might provide us with a bit of breathing room to figure out what works and stockpile supplies. But even if not I'd rather get this thing with the medical state of the art a month from now.
A vaccine isn't the be-all-end-all. We can still potentially hold out until there's an effective treatment. Famotidine? Remdesivir? Anything that would reduce the severity of the infection would lead to a better outcome than you're facing currently.
The point of flattening the curve is to keep the healthcare system from being overwhelmed, in order to avoid a catastrophic death rate.
With a vaccine still 18-24 months away, we're all going to get infected sooner or later no matter what we do.