“Agree that it can be confusing when switching to HTTPS results in traffic (and therefore probably also sales) going down. But I've read that this is a common short-term consequence and it should recover.”
It doesn't have to if you do it right, as i said in my earlier post:
...Basically you don't want to redirect your http website to your https one until Google has re-indexed all of your webpages as https, which is why I give the 6 month rule of thumb. This ensures any rankings you have (and therefore web traffic) is unaffected...
If you plan it well in advance and run HTTP & HTTPS in tandem then you won't see any drops or have any risk. Planning is critical. Those that have see drops in traffic have normally cut over too early or not planned or configured their sites correctly.