Well increased die size also helps cooling. Thus core count to GHz is not necessary linear equation.
You're right, it does, but increased heat output over a wider area is still increased heat output. The amount of thermal energy output by this chip is still higher. It's just no longer such a challenge to move it around. Ultimately your limiting factor is still the capacity of the heatsink to dissipate the heat once it's been conducted away from the die.
Let's say we have a true 100W TDP part with a 100mm square die area, no IHS and we put a cooler onto it capable of dissipating 200W.
If we increased the die size while keeping TDP the same, the temperature might go down a bit, because the heatsink could become more efficient, spreading the heat over a wider area of the fins, meaning less hotspots.
But if we increased that TDP to 200W, and compared the temperatures of the two, with the same heatsink, the temperatures would be the same, because once you hit the thermal capacity of the heatsink, it no longer matters how quickly or evenly the heat can enter the heatsink to be dissipated - the heat can only leave that heatsink so quickly once it's there.
The moment our example die hits 201watts of heat output, we enter a situation where no amount of die area would enable us to cool the chip, because the heatsink is simply unable to dissipate that much. It would be in thermal runaway, slowly getting hotter and hotter until eventually the chip hit TJMax and shut off.
So sure, die size helps cooling - but more heat is still more heat, and you still need a bigger heatsink to be able to dissipate more heat.