I just meant that risk increases with spell level. If that doesn’t work because some Nth level spells seem like they should be more/less risky than other Nth level spells, that’s a sign that the spells themselves are either poorly designed or should be a different level.
Oh I see, so you're saying since higher CR enemies are more dangerous, that spellcasters reaching higher level shouldn't make them any stronger than their foes.
Well I think the way 5e calculates save DC's is the problem here. In 3e, you used higher level spells not just because they had greater effect, but because they had higher save DC's.
So even though, say, Stinking Cloud was a fine spell, you might not be able to use it reliably against higher level foes, and be forced to switch it out.
Now that all of your spells have the same DC, you can get a lot of mileage out of your lower level spell slots- which rather nicely lines up with the fact that you don't get many high level spell slots to begin with.
But as I previously mentioned, the real problem isn't spells that deal damage, or buff allies, or even disable enemies. It's the utility spells that let you do things that effect the world in fantastic ways, which don't line up well with Challenge Rating.
I mean, the ability to turn an old keep into a difficult to invade fortress with one casting of Guards and Wards, or arm a village with a single casting of Fabricate can't be compared to concepts like xp budgets and relative monster strength.
Sure, some monsters don't care about such things, due to special abilities, but you find flying, incorporeal, and burrowing creatures (as examples) at just about every CR.