Currently Omicron cases are doubling every 2 days (it's more like 1.7 but let's go with 2). Today they reported 10,000 omicron cases (some were from yesterday, but let's go with 10,000).
At the same rate of infection, by the end of January, there will be 2.56 million cases of Omicron PER DAY.
That's why they are worried. Even if people are not as seriously ill, how on earth is life going to carry on with so many people affected at once, each isolating for 10 days? And of course some of those cases will be seriously ill and need hospital care. Hospitals will be over run with cases, there will be fewer staff to deal with them, and anyone attending hospital for ANYTHING will be affected.