I haven’t run a long campaign, and have yet to see these design principles in action, but this is how I “square” the imbalance you see here:
An average Attribute score is much, much higher than any one character Skill or Expertise. I know everyone knows this, but, for the sake of argument, let’s say that you’re advancing an Attribute, on the low end, from 7 to 8. It would cost 800 xp, then an additional 900 xp to go on from there, etc.
Now let’s say you want to increase a (relatively) high skill from 3 to 4 - 800 xp. Sure, 4 to 5 then costs 1000, but you could instead raise the Focus. In my view, you can “focus” on preferred Skills quite a bit more quickly and advance them to formidable levels—especially after, once the cost rises too high, you start building your Focus along similar lines.
Finally, if you still consider this an imbalance, remember that Attributes cap at 14. Skills (as far as I know) do not. In a long campaign, I suppose you could slowly raise all Attributes to 14 and only after this resort to raising individual Skills.