Whoever is the main commentator has to have the skills in order to do that though. Live cycling coverage involves potentially hours of not a lot happening.
You're over-estimating the knowledge of the people back home if you think a main man describing the action is not required.
Yes, Brian Smith and Magnus Backstedt could have a conversation about cycling for a few hours, but could they dumb down their knowledge to communicate and engage with the viewers at home?
So often there are no graphics showing who riders are. So often there are picture break-ups that leave dead air time. So often there are no time gaps. Harmon and Kirby etc are there to fill that time and stop viewers turning off. They do that by being entertaining, trying to engage people on Twitter, saying things that might be controversial, and of most importance, by engaging the expert in conversation about why things are panning out as they are.
The ideal I agree would be a main commentator who has competed in the sport in question, such as Peter Aliss in the golf or in F1 Martin Brundle. Even Hugh Porter at his peak, and Liggett turned down a career as a Pro to go down the commentating route. However, in most sports, it's a professional broadcaster who is in the main position. If a former sportsman can become the professional broadcaster, then so be it. As it stands, the only person in cycling that I can see doing that from the current generation is Dan Lloyd. He's doing the interviews for RCS events, co-commentating with Carlton for RCS and occasionally Eurosport, and presenting the online Global Cycling Network show.
It's a different world sitting in the co-pilot seat, not having to listen to producers in one ear and respond only when spoken to.