Surely only temporarily, though. I mean, this is an exponential process---adding one bit only doubles the space, and it will not take another nine years before some video passes 4 billion.
... and now I realize you may be correct, and that it's probably inevitable that a viewcount will not only exceed the total number of people alive, but will double or even quadruple it. Our total population is actually about 100 billion, but only ~7% of us are still alive.
The shadows of the dead will be forever enshrined as YouTube view counts. Our shadows.
"We always overestimate the change that will occur in the next two years and underestimate the change that will occur in the next ten." - Bill Gates
Edit: Now I realize that would mean Google couldn't have made this joke. But I am still not sure this was foreseen by Youtube devs from day one.
[One problem with unsigned integers in protocol buffers is that some supported languages like Python have no concept of signed/unsigned integers. Which is why for example Thrift does not support that distinction. This has nothing to do with C++ though.]
You can still implement range checks to enforce that the numbers are in the correct domain. It's not that much of a problem.
This is more of a problem in a statically typed language like Java, because it means there is no native data type to map protobuf's unsigned types to. In Python this doesn't matter as much because numbers will automatically promote themselves to bigints.