It's no longer possible to not be a nightmare. Companies no longer employ devs for life to keep the system functional and up-to-date, and (most probably) no devs would stay in such a role for their entire career.
When nobody knows how the system works, it can only be a nightmare: they keep it barely functional with marginal patches, accumulating debt until it finally crashes in such a way that it can't be fixed, then a new team comes in, designs and builds a new system, patches some major flaws, and then leave, and the situation repeats for another 10 years.
It's no longer possible to not be a nightmare. Companies no longer employ devs for life to keep the system functional and up-to-date, and (most probably) no devs would stay in such a role for their entire career.
When nobody knows how the system works, it can only be a nightmare: they keep it barely functional with marginal patches, accumulating debt until it finally crashes in such a way that it can't be fixed, then a new team comes in, designs and builds a new system, patches some major flaws, and then leave, and the situation repeats for another 10 years.