My second husband (the Cancer) was my best friend for 5 years before we got married. And I do feel that we'd still be together to this day, if he had not died. I still miss him, and I'm glad I married my best friend.
And yes, the Libra is my very best friend now. On a totally different level than my Sag BFF of 22 years, and my Virgo BFF of 30 years! Being friends with your lover opens a whole new world, and it's amazing!
Me & my husband defied like all the rules. Literally. We did all the "no-no's" in the beginning that most people warn against & yet we still came out strong, got married & remain HAPPILY married.
I get why so many people have a hard time transitioning from friend-stage to marriage though. Getting to know someone thoroughly is fun b/c of all the mysterious about a person each person gets to solve
BUT if you've solved all the mysteries & gotten to know them almost TOO well before the commitment, it kind of takes the fun out of it & leaves both people with nothing to look forward too.
So I can completely understand why even the bestest of friends for 5+ years never transition over to committment, let alone marriage.