Guaranteeing safety is absolutely a crucial part of it. And if you pull something out that doesn't work, giving people a false sense of immunity would also make things worse.

And safety shouldn't be underestimated. The story of dengue vaccine development is a cautionary tale. The first time you get dengue is usually not too bad, but it can be deadly if you get it again. Part of that is hypothesized to be driven by your own immune system, which, once immunized against one strain, produces antibodies against it in the next infection. However, if it's a slightly different strain of dengue the second time around, the antibodies meant to neutralize the disease are believed to make the disease more virulent instead (antibody-dependent enhancement). The immune system makes it worse. So when a dengue vaccine that didn't fully immunize against all strains was developed (not deliberately, people tried to immunize against all strains but this is a case where less efficacy than expected made things worse), it didn't make things better. It ended up priming people to get dengue.

Biology is super weird - even something as "simple" as vaccine development has a lot of unknown unknowns. And as dengue showed us, it's not just a case of "it might not work," because it could make things worse. That's why we have to run these clinical trials and take time to do things right, especially for something that would be as widely deployed as a coronavirus vaccine.