And in that time that it takes to turn and change course, Bitcoin could be down 50%.
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.
If you're long term enough to not care about a 50% intermediate drop then you're long term enough to never want to trade in the first place anyway.
You're not getting around the fact that
a) 1w MACD is extremely lagging (example: During the 2013 correction it turned red around $100. You had precisely 4 weeks before the market turned around, and your best buying back point would have been ~62. If you would have trusted the weekly MACD to buy back, you would have paid a nice 25% premium for the privilege of sitting out the bear market
b) it's pure momentum. At any moment, it can fail you, whether it's on the 2 min chart or the monthly. From a frequentist point of view, sure, I agree: once the weekly MACD turns green, it is more likely we continue to go up than down in the long run, but it's far from some logical kind of certainty.
Which brings us back to the original point: you don't base your decisions on a single indicator. The end.