The response was provided by Grinding Gear Games ahead of their 3.12 announcement on September 1. It should be noted that 3.12 is set to launch on PC on September 18, with consoles following on 23. But regarding Harvest League, the current 3.11 content, Grinding Gear Games explained why it will not become core Path of Exile content,
“When we designed Harvest, we wanted to experiment with a more deterministic crafting system that would enable players to access powerful items earlier in their character progression… As such, Harvest would require a significant amount of rebalancing in order to achieve a healthy level of power for the core game. We would be reassessing the crafting mechanics to determine how many of them we're willing to keep and how much of the remaining crafting would need to be rebalanced.”
Along with these issues, Grinding Gear Games states that the core mechanic in Harvest would create database issues with respect to the amount of data being stored which they say is a “non-trivial technical problem.”
So there we have it.