There are some good reasons why Android phone manufacturers have had a mixed relationship with removable SD cards. Some carriers stopped supporting removable SD cards only to re-add support; and Google’s Nexus devices do not support them while Android itself has changed the way that the cards are supported in ways that enhance security but have confused some users and broken some apps.
It would probably just make the most sense for phones to come with sufficient internal storage to begin with–32 GB should be the minimum, with 64 GB or more the norm. The flash memory used for phones’ internal storage is generally faster, and more reliable the flash memory found in SD cards. There are security problems with SD cards, usually due to the lack of fine-grained permissions on the file systems they use. (SD cards often use crappy lowest-common-denominator file systems like FAT32 to ensure wide compatibility across many different platforms.) Users have trouble dealing with SD cards (e.g., “moving” apps to them and having to tell individual apps where to store their data leads to confusion) and the behavior of apps and the OS when a card is removed or changed is unpredictable. Plus, carriers and OEMs have observed that most users don’t even take advantage of the ability to expand a phone’s storage with an SD card–most of them either never add a card or just use the one that may have been pre-installed.
On the other side of the ledger, there is no cheaper way than an SD card to massively add to a phone’s storage, and the swappability of cards is itself a feature–for example, you can just give someone a card loaded up with data.
I think there should be a compromise approach, modeled after the way that Apple does things with Fusion drives on some Macs. On an iMac, for instance, you might want the large amount of storage associated with a hard drive but the speed of an SSD. Fusion drives put the most commonly-accessed and working data onto an SSD and everything else on a hard drive. The two drives are presented as a single unified drive to the user and moving the data around happens entirely behind the scenes. Fusion drives are only slightly slower, and often indistinguishable, from pure SSDs. (There have been devices that operate under a similar principle to Fusion drives before, often single devices, but Fusion drives are distinct in that they use real SSD drives–not a small amount of SSD cache–and because the drives in question are just off-the-shelf parts, not custom hardware.)
Smartphones could be structured with a single unified storage pool, where 16 or 32 GB is high-quality internal NAND and the balance (say, 64 GB or more) is a microSD card. Data would be stored and moved around as appropriate on the two locations behind the scenes. This would greatly add to the storage available on a phone, and eliminate the difficulties associated with maintaining separate drives, mount points, and so forth. It would also remove the need for a user to have to manually decide what data to store where.
The downside of this is that it would be impossible to swap out a micoSD card that is used in this way; it may even be advisable for the card to just be located on the phone’s logic board. You might be able to upgrade the phone’s storage but only at the cost of a factory reset. However I believe the advantages outweigh the disadvantages, and the some of the benefits of swappability can be obtained if any phone that has pooled storage of this sort also supports USB OTG, so that they can read USB drives. In fact such a phone might still have a microSD slot, but one that could only be used for media and data (that is, apps couldn’t run from one or store their caches there, to avoid the problems that occur when SD cards are removed).