Your words:
The iPhone 6s ships with 128GB. 10+ times that amount would be about 1TB.
You can get ultra portable HDDs with up to 3TB that are way cheaper, more reliable and without the cumbersomeness of micro cards and flaps.
What's the point of using SD cards if you never swap them out? When you buy removable storage, it's to be removed, right?
Who said you had to buy the 16GB model? My first iPhone had 32GB and I won't bother with base models which don't fit my use (and I assume most other users). I understand why Apple sells those but I wouldn't recommend them to anyone.
I cant believe you aren't grasping how silly you are sounding.
an internal microSD slot means that you can currently store up to 512GB of additional media in your phone. Thats Movies, Games, Recording, Music. etc.
IN YOUR PHONE
without ever having to carry multiple devices. Your advocating that "just bring a WD passport" with you. Means you're carrying around not only your phone, But a portable hard drive, with it's wires to use it.
How is that in any way more convenient than internally accessible storage inside the phone? or better?
The BONUS on top of that, is you're not limited to just 512gb. You're limited to, however many cards you want to carry with you. They're tiny, yes, a downside is losing them, but an upside is, you can store dozen or so in your wallet without even noticing.
That means, you're carrying with you, Multiple TB of information, that can easily be swapped and accessed inside your phone.
With modern phones like the S6/S7 being able to record in 4k, this means you can now go hours of recording on memory card.
Not something you're going to be doing with your phone + portable drive.
I get that this isn't somethign that everyone will find advantages of. But it's by no means useless. You also don't have to use a memory card if the default 32gb is sufficient. If you do happen to run out of space on 32gb, you don't have to buy a new phone, you don't have to worry about deleting things, you can just buy more memory for fractions of the cost of a new, larger phone, and just add it in.
To argue that this is somehow a bad thing is insane, and indicates you really don't have the knowledge to be making the claims you are.