Indeed, that’s one annoying thing I miss about my new phone, as it doesn’t have a micro SD card slot. Another thing about new phones and this “everything cloud” point of view is that it’s becoming increasingly difficult (for me) to plug a USB drive/stick as a temporary ad-hoc storage device. So in addition to not allowing lots of space on the device itself, and removing micro SD card slots, it’s also becoming difficult to just plug a USB stick in an OTG port.
However, MP3s are not that big and anyone used to streaming shouldn’t bat an eye on compression. The loss on files compressed at 192 kbps is acceptable and you can have thousands of files for a few dozen of GBs. Also, when I started to “keep my files”, it was mainly in SD. Those files are perfect for devices with small screens and they are still small enough to be “portable”. A whole season of South Park is like 2.5 GB and my video player won’t tell my it isn’t available in my country. For 10 GB I can have 4 whole seasons on my phone and because the screen is pretty small, quality will still be more than acceptable. So, there’s still wiggle room even if phones will not allow TBs worth of storage.