I have never been a proponent of programs such as “KDP Select” (which I affectionally term as “SeleKt” on Facebook) or Kindle Unlimited, because these programs require an author to give exclusivity to Amazon. When asked by other authors, I recommend against it.
For me, this is a matter of mindset and principle, rather than a short-term view.
An appropriate comparison/analogy would be the principle of “buying American.” Many people prefer to spend money only on items bearing a “made in USA” tag. This can be something as minimal as a t-shirt, or a major purchase such as a car.
In the analogy, going outside of the principle and purchasing the t-shirt (or the car) from a foreign source will probably be cost-effective in the short term. A few pennies saved, or a few dollars at the end of the month when the car payment is sent off. Nor will purchasing one t-shirt (or one car) collapse the American economy. Just as with Amazon, a KU or KDP author may realize a few extra dollars in the next 30 or 60 days.
But, where the principle comes in: in the long-term, a mindset of buying foreign products serves to hurt the economy, taking away from American workers. In the same way, catering to Amazon as an author community serves to hurt the same community as we continue to give business to an entity that doesn’t really care about us. Indeed, Amazon has been abusive to talented, award-winning authors such as Nikki Sloane, Jordan Silver and others, censoring their works and forcing them to sell on other platforms.
This is why I always suggest saying NO to exclusivity with Amazon. The few pennies in the short term aren’t worth the detriment to the author community as a whole in the long run.