If this happens, will it be a good thing? If not, why not?
It's difficult to say. There are definite downsides to such a situation - for example, consider a scenario where there is a problem and two potential solutions, either of which would work, and the community is fairly evenly split between them. It doesn't really matter which is chosen, but neither will be chosen because anyone trying to push forward their idea will be pushed back by the other side. In the past, it's always been possible for someone in authority to step in a say "Ok, this is how it's going to be" and people would accept that. That is becoming less and less the case and sooner or later, we will end up in an argument we can't get out of. The most likely next step would be voting and decide policy that way. It would be the end of consensus driven decision making and the beginning of democracy (I think that is generally accepted as a bad thing).
That's the main problem with large groups - consensus becomes impossible to achieve. We've already had to switch to "rough consensus" in most places, which causes no end of problems since there is no real definition of what "rough consensus" is.
There are, of course, upsides - you've covered most of them, I think.
The only idea I've had for dealing with this situation once it gets unmanageable is some kind of parliament. The community elects a certain number of MPs, and the MPs make policy decisions (just making policy - enforcing policy in individual cases remains with the community) based on consensus. Basically, mixing democracy and consensus. It is a far from ideal solution, but it is getting harder and harder to make policy decisions, and sooner or later it will become impossible and we will need something.