Just celebrating them reinforces the traditions. Things are changing a little in America, not sure about other countries, but still for the most part Jewish holidays are celebrated without a lot of decoration and hoopla. Chanukah is the exception, because it competes with Christmas, but even so the to do is usually nothing to the extreme of Christmas.
On Chanukah, even if we are not very religious we light the candles, sometimes say a prayer, might tell the story of the miracle of the oil, and eat special foods. In many families gifts are given to the children, but not necessarily among the adults, it is not the massive commercialism of Christmas typically.
On Passover we read from the Haggadah which tells the story of our people being freed from slavery. We eat specific foods that night that are symbolic.
On Yum Kippur we ask for forgiveness, it is a day of atonement. A day to be pensive about ones life. You are supposed to fast, and then break fast the next day is usually a large feast when people get together with family and friends.
So, I think that the holy days have maintained their seriousness helps us to connect to our past, rather than it becoming overly Americanized, which to me is sort of synonymous with commercialized.
I am not religious at all, but I feel a bond with other Jewish people in that my people have suffered greatly over time, we have persevered, we enjoy prosperity now, and even with our very very small numbers, we hold onto our holiday traditions to be connected with our people past and present.
So many Jews are not religious that without these celebrations on Holy days the religion might fade more and more over time, and most Jewish people don’t want that to happen. They feel the way to get back is to live, to continue.