I admittedly have very little professional experience with SQL (just single table databases with SFW queries and inserts) but I've been learning more lately, and I find I strongly prefer writing it with lowercase keywords. It's easier to type since I don't have to keep pressing caps lock or holding down shift, and I don't find uppercase keywords to be any more readable (but like I said, I don't have much professional experience, and maybe it's different at that level) largely due to the syntax highlighting that pretty much every editor of the last 20 years provides. The only reason I can think of for using uppercase keywords is if you're working with a legacy codebase with existing SQL code that uses uppercase keywords.

It's not really an argument per se, but one other point I'd make is that so many programming languages have been invented over the years and afaik pretty much none of them use (all) uppercase keywords, in fact the only mainstream language I can think of off the top of my head that does that is COBOL, which is older than SQL. Did the creators of all those languages make a mistake by using lowercase keywords?