This is a misunderstanding of what Watterson was (is!) trying to achieve. He wants Calvin and Hobbes to exist on its own terms. You can call that a "brand identity" if you like, but it is certainly very deliberate on Watterson's part. What he is really trying to avoid is taking away the experience of reading the comic strip by having to make elements of it too concrete: What Calvin's or Hobbes' voices sound like, making the fuzzy line between Calvin's imagination of Hobbes and the real world too clear, etc. This isn't speculation, you can read this in the 10th anniversary Calvin and Hobbes book, which has a lot of great commentary.
As for how he was able to financially justify this, you can read about it in his commencement speech at his alma mater of Kenyon College, which I highly recommend reading in its entirety [1].
> Creating a life that reflects your values and satisfies your soul is a rare achievement. In a culture that relentlessly promotes avarice and excess as the good life, a person happy doing his own work is usually considered an eccentric, if not a subversive. Ambition is only understood if it's to rise to the top of some imaginary ladder of success. Someone who takes an undemanding job because it affords him the time to pursue other interests and activities is considered a flake. A person who abandons a career in order to stay home and raise children is considered not to be living up to his potential-as if a job title and salary are the sole measure of human worth. You'll be told in a hundred ways, some subtle and some not, to keep climbing, and never be satisfied with where you are, who you are, and what you're doing. There are a million ways to sell yourself out, and I guarantee you'll hear about them.
It's funny to me that all these years later, Watterson is still seen as some mysterious recluse with obscure motivations, when it couldn't be more clear: Watterson does not want to compromise his artistic vision, and he does not need more money than he already has in order to live a life he is content with. So there is no reason for him to milk Calvin and Hobbes for all it's worth.