it's not even a half feat though. It's a full feat.
Of course, expertise is a full feat, and with the extra proficiencies is a good choice for a feat.
But the Players Handbook feat "Skilled" that grants three proficiencies is a horrible feat. It is an example of how unsure the 5e designers were about how powerful the skills would be out in the wild, when players tried to abuse skills, thus made the feat too underpowered in an attempt to play it safe.
But later skill feats like Xanathars "Prodigy" shows a clearer assessment of what a proficiency is worth, balancewise.
I myself would consider taking a half feat that granted me four proficiencies of my choice, but not three.
Likewise, I would consider a feat that granted me three proficiencies plus expertise for any skill that I have.
I wouldnt take a feat that granted me eight proficiencies, I dont think. But using a proficiency as a kind of currency to purchase features is useful enough. Meanwhile, 1 proficiency ≈ 1 hit point. If the only way to get a certain proficiency was to permanently give up a number of hit points for it, what would be the maximum number of hit points you would give up? Your answer is the "real" value of a proficiency. This evaluation works best for the lowest tier features, available at levels 1 to 4. Suppose a hypothetical 5 hit points, plus an extra 10 hit points that you can spend on features or keep as hit points. Higher tiers can extrapolate from the solid evaluations of the lowest tier as the foundation for comparison.
Some skills are more powerful than others, but that is a separate complication. One can update the skills to make them more consistent, or one can rank the proficiencies, so that some are worth ½, 1, 1½, or 2. Darkvision, cantrip, or Perception is worth 2. (Of course, some cantrips are less good and worth less points.) Buying three weapon proficiencies is about the same power as all of them. So one simple weapon is worth ½, and all simple weapons is worth 1½. One martial weapon is worth 1, and all martial weapons is worth 3.