Little thought I just had

The series 3 finale places the end of the universe around 100 trillion years away. It seems to pose heat death with the total emptiness of the universe. However, the problem is that 100 trillion is dozens of orders of magnitude too young for heat death. Modern estimates place the universe’s total lifespan between 10^75 and 10^100 years into the future.

My theory is that the Doctor lives for around 100 trillion years all said and done, a truly ancient being by the end of his lives. Once he finally dies, the universe will also begin to rapidly decay around him. He is such a significant and fundamental part of the web of time, that it cannot function in his absence. Kind of like anchor beings from the MCU.