Hard to blame them for thinking that at the time. CD-based consoles had a very rough start in that era, but the PS1 was probably the first actual hardware success that used the CD exclusively as it’s medium. Nintendo had pushed cartridges to their absolute limit with the technology they had, so it was only a matter of time before someone ended up succeeding where others failed.
Survivorship bias. The ancient stuff that survived to the modern day are not more durable than contemporary engineering, they’re just the 0.1% of structures that managed to survive this long.
The problem isn’t that we can’t build something that will last a millennium, it’s that we rarely, if ever, need things to last that long. Nuclear waste storage facilities are the only thing that comes to mind. Everything else would need to be torn down and renovated or brought up to code at some point.