Or for each stat they could have a sort of table of "expected bonus to that stat at that level" - then take the percentage of that expected bonus that you actually have at your real level, and multiply that percentage by the expected bonus of the capped level. So for example, if they expect people to have +50 acc at level 75 and +20 acc at level 50, then if someone has +40 acc at level 75 (80%) they'd get +16 acc (also 80%) at level 50.
Or if someone's super awesome and has +80 acc (160% expected), then they get +32 acc at level 50: still not game-breaking, but in line with their super awesomeness and having gone above and beyond in the first place. Or alternatively if they had more than the expected bonus, they could simply *get* the expected bonus, but not get anything more than that.
It would, of course, require a bit of work and statistical analysis (or, heaven forbid, a rigorous itemization system in general), and it would have to somehow account for differentiating between uncapped gear (which should give its exact stats) and capped gear (which should be scaled), but it's something *one* person could do in a week in their spare time, even counting the time to punch in the item data into some sort of spreadsheet. SE's programmers would have the item data in databases to begin with, and they'd be paid to do that, so I'd imagine it would take even less time.
Again, that's another idea, which would be more effort but be more balanced than linear scaling. It does, of course, require effort - which gets back to my point that regardless of what a good system would be, it's plainly clear that SE was very sloppy and lazy about this, and didn't make anything that even *arguably* makes high-level gear worth using.
EDIT: I should probably add that this system doesn't piss me off as a player who'd intend to use it *points at that big "retired" over to the left* but as a programmer who's rather insulted by "professionals" who spit out shoddy programming. (Long-lasting chip on my shoulder, since of course you see that everywhere - Windows comes notably to mind - but it still bugs me.)