And: on the 'r' side of the r/K reproductive strategy. Whales are literally the exemplar of K-selection, that is a very small number of high-quality offspring.
Whale lifespans are long, populations and fecundity / brood sizes are small, sexual maturity relatively late, and childhood mortality relatively high. All of these make for slower rather than more rapid evolution.
Species such as krill (on which many whales feed) are far more likely to evolve rapidly in the face of increasing selection pressures. Whales might well find themselves boxed into an inescapable evolutionary corner.
Evolution of small things like algae and the krill which feed on it and feed the whale is quite fast. Single celled organisms reproduce on the scale of 20 minutes and hold immense amounts of genetic diversity in their populations to facilitate the success of a better adapted line almost immediately. Additionally, they are adept at horizontal gene transfer from other well-adapted organisms.
This would be great news if the whale literally only required krill to survive, but complex megafauna have complex needs, so the ability of krill and other small creatures to evolve is largely irrelevant in a discussion regarding the ability of megafauna to survive. This is especially true if you read TFA and see that the whales already adapt to eat different things as necessary.
It could easily become this fast or even faster, if we would just stop worrying so much about "playing god" and focus instead on getting good at this job. We don't have much time for this either, as AI is on the trajectory to take over that mantle in the next decade or three, whether we like it or not.
But seriously, we may not have much choice. Natural evolution stopped being able to adapt to environmental changes after it created us; genetic engineering is essentially the only way to make biology adaptable enough again.