That's not really the friendzone.
If I may, I'll use an example: I have male friend that I've been close with since before puberty, basically. Over the years (its been about 12 years) he has developed feelings for me. I have thought about being with him, because we'd probably have a fantastic relationship and there's no doubt that he'd be a great partner. However, he does not like discussing current events, research, history, engineering/innovation, or more "in depth" topics that I enjoy having a healthy debate about. I really value "intelligent" conversation and I've tried talking to him about it- it just doesn't interest him and he doesn't have much to say. My last partner was somebody who I could sit with and discuss anything for hours, and I really love that in a relationship. I learned a lot and we were like a sounding board of thoughts and ideas for one another. ANYWAYS long story short is that I still enjoy his company and love his personality, and we have a lot of fun together, but I have always told him with zero uncertainty that I do not want the relationship. He chooses to remain friends, as do I because I don't want to lose this friend that I've had for half my life, just because I'm not interested in a sexual relationship. Meanwhile he's very fit, handsome, smart, successful, etc. He's just not for me and I don't "lead him on".
Secondarily, he would never be a "backup" choice. I'd rather be alone than be with the wrong person for me. I just value his friendship and I'd hope that he values mine.
Tldr: you're thinking about it the wrong way. If you value a person in your life you're not just going to give them the boot.