It's ridiculous to say that if I buy an 8 channel preamp / sound interface, it's going to be obsolete in 3 years.
Assuming the unit interface and the computer don't fail, then there is no reason why it shouldn't work for ten years or more -- if you don't change the complete system in any way.
However, if you follow the popular bandwagon of 'upgrading' the computer or its OS there's a good chance that a new OS and/or the hardware involved will render the drivers incompatible and thus the interface becomes obsolete.
It's kinda in your own hands. I still run an XP machine to retain compatibility with some favoured decade-old hardware that would otherwise now be obsolete...
Maybe a line only interface is the way to go.......
But an interface to what? As others have said, it's not the digital audio interface that you need to worry about. AES3, S/PDIF, ADAT, and analogue will be with us for decades to come.
It's how you get that audio into and out of the computer that is the issue. Network-based systems like Dante would seem to have the best long term viability, but you're talking ~£2k for a computer card and analogue break-out box at the moment -- even for just eight channels of I/O.
So maybe it is actually more cost effective to buy a cheaper USB interface and budget to replace it in five years time when the computer needs upgrading...
Technical Editor, Sound On Sound