Biweekly is not its own antonym but it means two completely different things (every other week and twice a week) which for me as rendered it useless since you cannot know which meaning is intended.
Yeah the RAE is super useful to end debates on what a word means, thats it. I see no issues with language not "evolving enough". Spanish written/talked a century ago is different than what it's spoken presently, even if the words mean the same.
The best way to deal with this issue is to have body that slows down language changes, then normalise them based on logic and history, something like the https://en.m.wikipedia.org/wiki/Royal_Spanish_Academy.