The PSTN doesn't offer guaranteed end-to-end transmission, and certainly statmuxes based on expected load. Looks like similar capacity planning.
The PSTN does guarantee a certain service level, latency, call completion etc.
Perhaps you refer to latency. Most people don't care as long as HTTP and POP3 latency is "good enough" -- and server response time is often a substantial consideration. SMTP really isn't picky about latency or jitter.
Latency & Jitter are very important when dealing with sound & video. Or anything realtime for that matter. The Internet isn't just HTTP, NNTP, SMTP any more.
Maybe you mean packet loss. Most everyone here can recall the days of 30% packet loss across congested MAE FDDI fabric, but that went away what seems like eons ago.
I remember quite a bit of packet loss when the last series of worms hit