I agree, it's a common request and I've wanted the functionality myself quite a few times.
TheBrain is a bit different from a Word Processor. Each thought is a record in a database, sort of like it's own document. So currently TheBrain behaves like a word processor, you can't do a find and replace on multiple documents (or in the Brain's case, thoughts) at the same time, only the document you have open. Doing a find and replace across everything in a database is a bit more complex as you have to make sure you don't break references in the rename, as well as specify which parts you want renamed, and finally, make it clear in an interface that there's difference. Can you imagine the problems that would arise if I renamed "Alice" to "John" and thought it was only in my notes, but it went through the entire database and made the change? I might not know, and come across it months later and realize I've just mucked up my data. Making it extremely visible and understandable to the person using the software is just as, or perhaps more important than the rename function itself. I'm not trying to make excuses or say your request isn't valid or important (it is, and I'd really like it too), I'm just providing a bit of context around the request and an outside perspective of why it may not have been implemented yet.