The idea that any self loading action remains clean is a serious mistake. However it gets driven, an action that extracts a cartridge from a chamber under it's own power - gas or recoil - will have residual gas blown back into the action no matter what. Only manually operated weapons keep a clean bolt face.
The second mistake is to consider the residual buildup in the action as a major cause, or for that matter, even a potential cause of stoppages. The modern TDP for the M16, with chromed chambers, proper dimensional checking, proven first use powders, and the REQUIRED lube of the cam pin track all contribute to keeping the M16/M4 running. If anything, it's been demonstrated time and again that AR's will run for tens of thousands of rounds without cleaning. Getting dirty will not stop a gun built right. It will keep firing, and the AR demostrates it time and again.
It's when all the wrong things are done than we hear about malfunctions. Going back to the dust tests, if poor maintenance and bad magazines are used, you get poor results. Use weapons and mags in good repair, they shoot better. Please remember the majority of stoppages are from magazine and ammo related causes, and many of the anecdotal stories on the internet involve low power ammo and weapons built without any TDP at all other than a bulk order for all the parts.
For all the complaints DI runs dirtier, no one explains what causes a specific problem that actually stops the action cycling. The bolt carrier and bolt haven't been changed in 45 years to "improve" them or make them more resistant to fouling, and yet that's the #1 complaint brought up in theoretical discussions. When actually tested, it falls far down the list, with mags and ammo sharing the bulk of the severe problems up front.
Just exactly what is it that causes the bolt to jam? Pics, please. Link to an official report listing just how much x.xxx inches of residue built up on the operating parts causes it to become a sludge pile. Finger pointing and namecalling don't make for repeatable results in failure, and again, back to the dust tests. The majority of problems are mag and ammo based, not residue buildup, even when a lot more is added beyond the gunpowder.
It hasn't been a problem since 1968, ask those who have been shooting at us what they think about it.
If we are going it have a better weapon with better subsystems and parts, it's going to take a lot more objectivity, study, and understanding of what it does. Simply repeating street talk and unfounded rumor won't get us there.