Yes indeed, the CO2 concentration has been higher.. with a Very Different ecosystem (flora/fuana)

then...to reproduce that environment would have dramatic effects on our current lives...plus the changes there happened over tens of millions of years...not centuries or decades...IMO this return to the pre-historic regime would be catastrophic and chaotic for humankind...the data is not ambiguous...we're looking at a truly 'sudden step change' in our environment...not something to be trifled with...especially when there are actions we can take to significantly minimize it.