Why on earth *not* commit to your friend as a platonic life partner, if you two really get on so well? After all, one primary doesn't preclude another. If/when you find the romantic primary partner you dream of, that person can be co-primary with your friend.
Of course, this thought is predicated on you and your friend really caring enough for each other that you'd be comfortable building a life together, and maybe that's not realistically the case. But if your hesitance is because you think only one person can ever fill that "slot" that's just not the case.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.