The RC4 system has inherent issues that were documented in Jira, plus it also has a systematic risk to the network (Sporking is less risky than a hard fork, but there's a risk the network forks wouldn't actually go away when the spork was turned off after a failure). These few issues combined with the fact that we've reached 80-90% payment efficiency tells me that it's not worth the risk to the network to move from the RC3 payment system.
Also, I have a separate plan I've been considering as an alternative that carries no risk and can be done at RC5's launch. Basically, I would set a minimum protocol version and boot anyone not running RC4 or later off the network. This should get us to 98%+ payments.
why 98%+ and not 100%
Why
What's 2% when we'll touch the sky?
Stand forth here now and salute heir yonder,
Forget the past, he'll bring us a wonder....!
“Yes, a Jedi’s strength flows from the Force. But beware of the dark side. Anger, fear, aggression; the dark side of the Force are they. Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan’s apprentice.”
(people pay for advice like this) 
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