Re: So Close!
"When phones only had 4 or 8GB of storage, an SD card would be a 'must have' feature. However, in these days of phones with 128GB storage, it isn't as crucial to as many people"
The SD card surely had its benefit when the phone memory is low (4 or 8 GB), but today it still serves as an alternative to local backups and quick data transfer. Imagine you had a 100+ videos collection at close to 128GB, using cloud backup restore or even phone wire to transfer the data will feel like age. But with an SD card, you can transfer to the SD card while you use your phone and plug it in when it's ready. Of course in reality it will be less than 128GB, but you get the idea.
"Some apps will by default store to an SD card, but if the card is removed"
I can't say for sure but I haven't see apps auto move to SD card for some time with newer android, but back in the days of early android it does that for some branded device with low storage. If it does happens now a day, a simple fix is to install the app without the sd card and insert the sd card back later.
"In WhatsApp this makes some data permanently unavailable"
You might want to look for the Whatsapp folder and backup everything there. If you done it right, you should now have a backup of a folder called Databases with some whatspp message files, and a backup of a folder called Media with all your whatsapp documents / photos. If you reinstall the WhatsApp app with the whatspp message files in the right location, your messages should return back to the way it was with your 'data' available.
The SD at most would break the WhatsApp app but not the backups, unless WhatsApp never created a single message backup by itself.