The Second Amendment does apply to knives. Know the knife laws in your state, and in other states you may find yourself in.
If personal protection is a priority for you, a knife is something that you can use right away, without having to identify yourself or register the weapon.
I’d argue it’s a hell of a lot better than useless even after the fight starts (especially if it’s unarmed), but would agree that spray is the compromise between a strong word and ending a life.
I’m glad to see people talking about options. Not every tool is going to suit every person’s needs or wants.
I also have a nice camping knife with a ferro rod. I can process wood and start a fire. I can clean game and fish. It’s big enough that it could put someone off getting too close, but yes, I would have to be committed to drawing a lot of blood.