While it truly takes 'four to tango', the quartet's failures and successes overall hinge primarily on the lead's ability to take command of the ensemble during a performance.
This isn't to take anything away from all the legendary basses, baritones, and tenors throughout the history of our Society, but when it comes down to it, isn't it a lead contest?
As a lead who constantly struggles to improve his own feeble skills, I often ask myself, "If I were to step down from my current OR previous quartets and be replaced by one of the legendary leads in their prime, would the net gain be bigger than replacing any other part with comparable talent?" I think yes.
This holds true in secular music as well. Just imagine U2 without Bono... The Stones without Mick... Led Zeppelin without Robert Plant. Queen was never the same without Freddie Mercury. And we've seen Van Halen without David Lee Roth. Not even Eddie Van Halen's guitar magic could return the band to its previous heights under Diamond Dave's tenure.
So to the leads out there, stay strong! And as my hero Mike Slamka says, "Sing it from the heart, or don't sing at all."