His strength as an advertiser has always been his ability to empathize with people and sell them what they really want. After his life fell apart, and the money and products he thought would bring him happiness weren't enough, being another company guy grew tiresome for him and he had no idea what he was selling. By finding that inner peace, he now can empathize with the existential crises people go through and what they need to be happy, and can use that to be a better advertiser.

Don finally overcame his own issues which enabled him to land the Moby Dick he's been chasing his whole career. It's more believable than him attaining that peace and then repenting for his career of misleading people while making his children vegan pancakes in sunny California two times a month. He filled the hole in his life, and now, after making a huge breakthrough in the advertising industry, he can happily go back to doing what he does best.

I was very impressed and surprised with the finale. Without that Coke ad it would have been too Hollywood for me.