That doesn't matter because we're talking about long term trends.

If you're investing long term, you don't care if it goes down 50% as long as it comes back up again. That 1-Week MACD histogram doesn't guarantee that the price won't drop by 50%, but it does guarantee that such a move would be the equivalent of swimming against a very strong tide and is likely to be swept away by same tide in quite a short space of time.