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> ...until every Mac is updated to High Sierra...

Aye, there's the rub. I have a number of older systems, still in active use, that won't run High Sierra. The ability to transfer files between newer and older systems -- using an external USB SSD or USB Flash drive -- is critical.

If I insert such a drive in a High Sierra system, and it automatically reformats it for APFS, it becomes useless to me.

Oh well.

If I insert such a drive in a High Sierra system, and it automatically reformats it for APFS, it becomes useless to me.

Ah, but dayglored macOS High Sierra DOES NOT automatically reformat any USB drives or external SSDs to APFS that are just inserted in, or connected to a High Sierra Mac. That's not what it does.

A computer running macOS High Sierra can read and write to both APFS and HFS+ formatted Drives.
A computer running macOS Sierra or an older Apple OS, including all Apple OSX, can only read and write to HFS+ formatted drives. Computers running these older Operating Systems cannot read or write to any APFS drives.
A computer running Apple macOS High Sierra does not automatically format any USB or external SSD or Hard Drive to APFS that is just merely connected to that computer.
The only drive that will be automatically reformatted to APFS is the boot drive of the Mac where Apple macOS High Sierra is being installed.
Any subsequent USB drives or SSDs that will formatted using the DEFAULT APFS formatting command. The ability to format in HFS+ is still available in High Sierra.

The limitation is that macOS Sierra and older OS Macs cannot directly read from or write to the upgraded macOS High Sierra APFS formatted drives. . . but the macOS High Sierra computer is not limited from writing to and reading from standard Mac HFS+ drives either internal or external, or on other Macs.

This is actually the second time Apple has had to do this. The first time was when Apple moved from the old Apple file system of MacOS to HFS+ back in January 1998. The complaints were similar then because the old HFS could not be used for booting. . . but it had to be done in one fell swoop.