triple A titles are part of 'modern gaming'.

They are modern gaming, but they are not the only one. A lot of modern games can be played on potato machines and a lot of people don't need to upgrade. 4 cores era is only over, if new games need more than 4 cores to run adequately and the majority of players have moved on. Right now, it did not happen.

another reason why it's relatively dead is even if you were to build a modern system now,

If the whole discussion is about building a new pc, then I might have misunderstood it.

it makes no financial sense to buy a 9100K 4c/4t when a ryzen 1600 6c/12t is the same price

Not denying it. Building a new pc is a different thing than analyzing if "the era" is over yet.