puh leeze
Not really.
Think of the Great Plains 200 years ago, teaming with great herds of buffalo, deer and pronghorn, or East Africa 100 years ago, teaming with wilderbeest, zebras, impalas and Cape buffalo.
In both cases, open grasslands came with huge herds of grass eaters.
These people—probably the early Indo-Europeans—hunted vast herds of reindeer, mammoth, bison, and horses on the tundra prairie.
The tundra grasslands turning back into forest would have decimated those herds.
“A longer, more fruitful growing season was “challenging” ?”
Well sure, if all you know how to feed yourself is scavenging and hunting under certain conditions. BUT - the climate changes would have been fairly slow I would think that one could adapt.
Although perhaps not. The Vikings in Greenland left after years of cold weather ruined their crops (they were farmers by then). The natives offered to take them on fishing and seal trips. But the natives would hold their rituals ahead of the hunts, and the Vikings (also now Christian!) wouldn’t partake in their pagan worship.
Adapt or die. Or at least move.