The main specs that really matter in an interface for most folks are mic gain and preamp noise. (Failings which are most exposed using dynamic microphones on quieter sources). Converters have been good for a long time, though there are probably a generation of newer chips out there which may operate a bit faster, or have even higher frequency support, but for home recording, eh, I get by at 48kHz. (I'd still be using my 10+ year old Focusrite if my new iMac had a FireWire port and stated OS support!)
Short answer is if you're recording and it sounds ok to good, it's probably not in need of an upgrade.
Now, features might make a difference, and if there's a software app that enables more features, like DSPs or the like and that app no longer works, well, maybe that's a reason to move along. Or, you want better metering on the front panel, another set of inputs/outputs, whatever - all valid reasons to update.
<My2¢>Of course, if you spend a couple thousand on an interface, I have no doubt *you* will hear a difference., but its unlikely that any consumer/pro-sumer interface is going to make an [blind-test] audible difference.</My2¢>
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