Apple has been using USB for 19 years (they were the first major adopter, in fact). They're in the process switching to Type-C ports, but at most you'll need a new cable, adaptor or hub to connect your older devices.
Firewire was never particularly successful, but Apple kept it around for well over a decade, and again you can still buy an adaptor for modern Thunderbolt PCs.
SCSI? Really? We put up the terrible hardware for the speed and protocol improvements over early IDE, but SCSI products were always niche in the consumer world and the writing was on the wall by the mid-90s. But on the off chance you saved all your data on SCSI drives and then hid under a rock for 25 years, you can still pick up a PCI-e SCSI controller (and a PCI-e to Thunderbolt box if you really have to hook it up to your new Mac).
While Apple is relatively aggressive at removing "non-essential" ports and features, USB seems the least likely of all ports to be removed, and even then Apple would sell an adapter to get you through the next decade or more. Past that, you could probably find an aftermarket USB-to-new-whizbang interface for another decade or more. Hell, your Mac can still party like it's 1989 with an ADB adapter and Token Ring bridge; USB will be with us for a long, long time. Even beyond that, you could borrow an older machine to access the data; it's not hard to find a ten- or twenty-year-old PC; most Slashdotters have one in their house.
But this is academic; by then you'd surely have shifted your data onto a newer drive anyway. Drives fail, even when not in use, so if you care about your data you'll be maintaining a few copies on various media and spending a few hours per decade moving that data onto fresh disks. Compatibility should never be a hurdle.