One of the things I've noticed, in updating my saga to 5th ed, is the disappearance of Permanent as a Duration. Does anyone know why this is?
I'd say game balance.
Personally, the game got better with the new limitation.
My guess is the eliminations of headaches and confusion. What spells can be permanent vs. what spells can't be permanent.
Why make a magic item if you can invent a permanent spell to do the same thing?
Now permanent spells are restricted to creo rituals with natural effects. it's certainly eaisier but there are some issues with making the guidlines momentary. (I want to raise my sheild grog's strength by 1 point for duration diameter and you're telling me that it's a level 45 spell!?!? I can turn him into a brown bear at level 30!)
AFAICS it is also a consequence of the 5th edition limit of essential nature (ArM5 p. 79f), which is phrased more precisely than in previous editions: Hermetic magic violating a thing's essential nature must be maintained, either by magus or magic item. A generally available spell duration 'permanent' does not fit with this limit.
Kind regards,
Berengar
So... real good reason at all. 
Permanent spells caused endless problems in 3e/4e - especially with the bizarre split of Instant into "here and gone, easy" and "here for ever but the magic gone, very hard"
5e clarifies this:
instant is easy, short magic (here and gone), but Creo Rituals can create real things which remain after the magic is gone.
No other spells remain longer than a year, and all those last only until dispelled.
In addition, 5e clarifies exactly how you build an "always on" device, which will create and sustain a magical effect.
Thus you can no longer create a permanent effect "in a trice" by spell-casting, even with rituals; but you can if you spend the time and vis in the Lab. (A single season will mke a Lesser Enchanted Device which can be Always On).
This seems to add considerable flavour...
I still find it a bit silly that The Touch of Midas is a Lvl 20 Ritual, but a non-permanent version of the same, with - say - Moon duration, would be Lvl 35 (though not a ritual).
Well, I guess it forces magi to be more honest... 
It took me a bit, but now I rather like it. It seems okay that it is easier to make something with vis that lasts, but harder to make something without vis which has duration. I don't know. It makes sense to me. (Plus it does at least have to be level 20 or something to begin with...)