When you find the right backpack for your needs - and the needs of your cat - there's absolutely no need to consider backpacks cruel. Indeed, they are actually ideal for cat fans who simply want to ensure their cats comfort and needs ahead of their own. Instead of keeping the cat in a hard case, which often needs to go into storage during travel, a backpack will keep your cat close to you. This, in turn, reduces the stress your cat feels.
However, there is one addendum to this. If your cat does not enjoy being in their cat backpack, then using a backpack is cruel. In the end, your cat's safety and comfort should always come first. And forcing a cat to enter a confined space, for your journey, can be unnecessary. In these cases, always try to find an alternative.