A very bad outcome (which we don't have enough information to rule out) is hospitals being overwhelmed with sick people, so that some patients are left untreated, with a greatly increased rate of dying as a result. The reason this is possible is because there are a large number of mutations in this variant, which might allow it to bypass people's immunity. This can make immunity acquired through previous infection or vaccination ineffective.
People that claim this kind of stuff "just makes sense" aren't taking into consideration the second order effects on human behavior that might make things even worse. For example closing outdoor parks probably made things much worse because people would meet inside instead...
Health professionals were factoring in second order effects. The problem is that there isn't a wealth of research or evidence which shows which option is more likely to work. So a lot of at least in the early days was trial/error.
What would do know is that increased mobility translates to increased cases. And so opening parks whilst much safer than being inside results in increased mobility as people need transport to/from those parks.
If you didn't have that quarantine they would be out in the wild infecting people. Not what you want when many parts of our country e.g. Western Australia had zero community transmissions.
Without some sort of travel bans there is no way you could manage that level of inflow.