NNTP servers were generally provided by your ISP, so ISPs built the capacity out as they added subscribers, and added groups based on the perceived interest. It wasn't especially difficult (and it's pretty trivial today) to offer all the "official" text-based newsgroups with a few days or weeks worth of retention; it only becomes a significant undertaking when you want to offer months of retention for binaries newsgroups.
Similarly, because the NNTP servers were basically a value-added ISP service, they were funded by subscriber fees. You never had a problem with a newsgroup suddenly getting popular and being unable to pay their own hosting fees, or suddenly tossing ads in order to pay the hosting bill, which is unfortunately common with webforums. And you don't have problems with a single server crashing and wiping out the history of a whole group. It's all pretty robust.
Architecturally, netnews was superior to web forums. Where it failed was in not having a good response to the spam problem. I think that had it not been for spam, people wouldn't have left NNTP for HTTP-hosted webforums nearly as quickly. But the decentralized nature of netnews meant there wasn't anyone who could quickly implement any of the myriad solutions that were proposed (cryptographic proof-of-work plus reputation systems probably would have cut down on the worst offenders).
My guess is that eventually, we'll reinvent something that's similar to NNTP but by another name (with the decentralization, server-server replication, gateways to other protocols, etc.), but it's been a long and unfortunate diversion in the meantime.