When I learn redis, I spend time and want to amortize that over a long period. When I integrate elastic search into my application, I expect to be able to use it in the same way far into the future.

Relicensing, as you point out, doesn't affect past versions, but it sure does future ones.

Now I have a surprise chore on my plate, to figure out if and how I need to replace the existing component or learn about an alternative.

More than that, my confidence is shaken. Will they make changes in the future requiring more work on my part?

Changing a license is very similar to an increase in price, but even more fundamental in terms of uncertainty. And people hate change.

(I'm explicitly not addressing the impact of a license change on software freedom because I think it is very important to some. But IMO most folks are more interested in free as in beer than free as in speech. I don't know enough to speak to the free as in speech aspect, so won't.)

I think you asked a great question, hope my answer sheds some light.