Sounds cruel, but on the longer run this is the best solution, and most likely the only one that can be successful. There are however a few downsides... Not only old and frail people can get seriously ill. Sometimes strong healthy people can also end up on the intensive care, and not all countries have enough intensive care capacities to keep the worst case scenario in mind, and this is a bit what they fear now. But in the longer run, in the basic theory that is the only hopeful solution we got, and the ones who did contract corona will in the eventually get immunity and that can protect the old and frail.
But here's the rub. How long does the immunity against corona last? Nobody knows that yet, as there's only one way to find out. Deliberately make somebody ill, and as soon as they recover keep checking if antibodies are still present in the test subject's blood. Maybe you get where I'm going. If it takes 1 year before the antibodies are gone, that we won't know until next year. If we know it any time sooner, then we got bad news, as that means that antibodies do not last very long. This knowledge is also crucial when it comes to the question if developing a vaccine is useful or not... If the antibodies last only a few weeks, developing a vaccine will be pointless...
For the short time I've heard that the drugs used to fight malaria and AIDS may, according to studies be successful in curing those who have corona. Particularly the AIDS drug (odd as it may seem) a very good candidates due to it having very little to none side-effects. Since these drugs already exist, they don't need the normal testing procedure to see if they are safe, so they only have to test if they are actually effective. I've heard a microbiologist say she was very optimistic about this development, but she impressed that nothing is certain yet.
What does seem sure is that locking down the world is gonna do more harm than good....
In Dutch we may say "De genezing is erger dan de kwaal" (the cure is worse than the disease).