Most squirrels are born in about the exact same environment as their great great great great grandparents. Some years they eat more berries and fewer acorns but many environments don't change much from century to century. In such an environment each successive generation should be more fit than the last since weaker, dumber, and slower squirrels are being picked off by predators. Indeed, almost everywhere even including changing environment this must necessarily tend to be true.
It makes no sense to dispute this.
Sometimes weak, dumb, or slow is adaptive, it benefits the organism.
And many populations
don't change much from century to century. But most environments do change, sometimes slowly, sometimes faster. Climate changes, biota mixes change, and living things adapt. Accumulated adaptations eventually produce new species.
A blind cave fish is more fit than a sighted cave fish. A legless reptile or marine animal is sometimes more fit than a quadrupedal one. A flightless bird is more fit, within its niche, than a flying bird.
Some of the most successful, best adapted, most fit "designs;" designs that have been around unchanged for hundreds of millions of years, are exceptionally slow, dumb, and weak. Think sponges, jellyfish, snails, turtles, &c.
It seems like it's the "high performance" creatures that are always dying out.