Good question. You may be surprised to hear that my stance isn't that uncommon in research. If I recall correctly, somewhere around 50% of researchers personally will not use mice in their experiments. In these cases, we would either use a lower lifeform (fish or fruit flies), or use immortalized cells. Immortalized cells are aggressive cancer cells that happen to retain some of their cell properties. For instance, immortalized lung cells tend to act somewhat like actual lung cells. It's not a perfect model, since you're experimenting on cancer cells instead of actual cells, but the ease and low cost of growing and using them makes them extremely valuable for a lot of grindwork experiments, where you just need to burn through tons of different hypotheses quickly.

For me, I prefer to use immortalized cells. It works out for me anyways, since I prefer to focus on the mechanism of disease (which tends to be easier on immortalized cells) rather than practical effects of disease (which tends to require animals).