But there will be no scaling, so there's no issue? Unless I misunderstand (which I probably do

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Well, the question was why spells had to become obsolete.
So I was pointing out that structurally, in earlier editions, it was bad for the game if a 1st level spell slot was doing the same work as a 7th level spell slot when your character was at, say, 15th level.
My general take on the issue as a whole: Its annoying as heck to have something you think as cool, and then have the game take it away. One way the game can take something away is the 4e way- to literally tell you that you forget how to do it when you learn something better. Another way is the 3e way- to let you continue to do it, but make it so that the thing sucks at high levels, and even though you CAN do it, you don't want to.
And there are a few ways to make it so that you can continue doing the same cool thing at high levels as at low levels. One way is to make the thing scale, but that creates problems. Another is to let you upgrade the thing so that you're doing the same thing in a way, but bigger and better.
4e did this by offering powers that were better versions of lower level powers. For example, a fire wizard might learn to throw a ball of fire at someone as a level 1 at will power. Later he might learn to throw a better ball of fire at someone as an encounter power, and an even better one as a daily power. Eventually these encounter and daily balls of fire would become weak in comparison to new enemies, but there was no shortage of new, better, higher level balls of fire to throw at people. So that wizard might lose his level 7 encounter ball of fire in exchange for a much better level 17 ball of fire. At all levels the character still "throws balls of fire at people," even though he doesn't strictly use the same spell.
3e did something similar. At spell level 3 you had fireballs. At higher spell levels you had things like delayed blast fireballs. And you had metamagic, which was another means of upgrading a lower level spell so that you could continue to do the thing the spell described, but while using a higher level spell slot, so that balance could be preserved.
In general, I don't care what system 5e uses as long as the goal is preserved. I was initially just commenting on why things worked the way they did in 3e and earlier.