Am I right in understanding that the relicensing was possible because of the CLA, not just because of the BSD license? Would a permissively licensed project that didn't use a CLA be vulnerable in the same way?
A CLA may impact relicencing, it depends on the terms. A simple CLA may only say "I am the owner of the code and I release it under $LICENSE". The current Redis CLA also has a copyright grant, which gives Redis the company greater rights.
And the problem with the trademark model is that AWS, and especially Microsoft, already have established brand recognition with the people who sign the big SaaS and support contracts. The people who know what a Redis is are just nerds with no money, the real big shots do everything in Microsoft Excel.
https://www.fsf.org/bulletin/2022/fall/copyright-assignment-...