One thing I think about with 4e, is that the real sameyness between powers is actually within the powers of the same class. I remember looking through powers to convert to 5e and thinking that some higher level powers just looked like lower level powers but with bigger numbers. I think it would have been better to simply allow lower level powers to scale every so often, it may have saved some space. This would have also been great for those powers that didn't have a higher level version but which you really liked for your build.
So you have up to 3 encounter attack powers from your class. At level 1, 3, 7 you gain a new one. Then st 13, 17, 23, 27 you get to upgrade one. Assuming you replace your lowest power, thst means:
1->13->27
3->17
7->23
are where powers are replaced. And similarly for daily:
1->15->29
5->19
9->25
Instead of cluttering L3 powers with upgrade rules, they put an upgraded version at L17. This is game wise nearly identical, but it makes the L3 rules a bit easier to read.
Like many things in 4e, it was designed to be
played not
read. You where intended to start at level 1, then adventure, level up. And repeat. When you got to level 13, you'd be told "you can replace a power with a new one". Your choice is likely to include a power that is a nice upgrade to the trusty level 1 power you've had all along! As well as some other picks.
So instead of "I don't want to upgrade, I like my level 1 power", you are told "well great, here is one you won't mind picking". Importantly, if you played the game a level at a time, you never had to know this was coming; you just ran into level 13, and it happened to be arranged to give you what you want.
For the first 12 levels, what happens at level 13 didn't matter. So there where no rules in the first 13 levels of stuff that tells you what happens at level 13.
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Now it also permitted double dipping. Instead of replacing L7 power (come and get it) with the L23 version, the epic fighter could keep both, instesd replacing a L13 power with a L23. They could do the same with a L5 daily at L19.
That resulted in some interesting system mastery tricks you can do.
Had they had upgrading powers, either they eould require you to burn a power upgrade on them (which is annoying to write the words for), or the free upgrade would have to be accounted for in character power budgets (balance math). They where trying to avoid quadratic wizard syndrome from 3e, where you gained a linear number of powers each of linear strength.
Each new upgrade gave you one unit of power upgrade. If low level powers auto-upgraded to be on par, you could bump a different power up and break the budget a bit. It would probably still work, but the concern is valid.