We don't, and apparently they didn't either. But it was their job to do so.
I think it's very reasonable to suspect underinvestment and technical debt here. Not only did they have a problem, they had a very hard time a) finding the cause of the problem, and b) mitigating the problem while they were looking for the cause.
We can't know, of course. But I think it would be hard to argue that this is optimal, and that nobody at CenturyLink ever suggested it could be better if they spent a bit more money.
It's hard to work on network problems when the management network is also having problems. That requires dispatching people locally to sites to see what's going on. You're blind in both eyes, not just one.
Look, I'm not defending them or their choices. I'm just saying that this more more nuanced than "They suck they should have known better or spent more money." When you run a network as large as this and with as many "legacy" technologies as they do, things aren't very cut and dry.
NTT probably has the best managed global network with their extensive use of SDN. They run a tight ship and everything goes through their automation frameworks. This requires a huge investment in R&D. Even then, it doesn't cover 100% of their network because "there's that T320 for that customer in Chicago, and the stuff up in Michigan for Dorian's T1." Multiply that times 30 or so to consider size differences between NTT and CenturyLink....and yeah, stuff happens.
But your last paragraph specifically describes technical debt as part of the problem. If even the best org (NTT) has technical debt and CenturyLink isn't the best org, then I think it's safe to suspect that a) CenturyLink has significant technical debt, and b) they have underinvested in that technical debt compared with industry best practices.
I think it's very reasonable to suspect underinvestment and technical debt here. Not only did they have a problem, they had a very hard time a) finding the cause of the problem, and b) mitigating the problem while they were looking for the cause.
We can't know, of course. But I think it would be hard to argue that this is optimal, and that nobody at CenturyLink ever suggested it could be better if they spent a bit more money.