I think too many people are thinking in a one-way implementation for this game design aspect.
The idea of the current system is, that a powerful special sword, isnt special and powerful because of the stat that gets downscaled to your level, and is instead special and valuable because of its special effects and combination of defining attributes.
Now I have no evidence this is what will happen of course, but its a logical and not totally stupid way to do it, and its good to consider it as a possibility, rather than automatically assuming whatever thing you dont like will also get implemented in the worst way possible so that it fails miserably.
If damage changes according to your level when you find a weapon, its extremely likely that damage will never be what makes a weapon legendary or very valuable, otherwise it'd make no sense, nobody would design something like that.
There are also other variables, such as damage downscaling responding to a percentage and not a fixed number, which would actually allow even damage to make a weapon special based on it. (you pick a sword at level 5, the sword gets reduced damage only 10% if its supposed to be especially damaging, unlike 30% or more compared to other swords)
The linear attributes that would create an imbalance because all swords have them and and some have to be better, such as damage, will naturally not matter much, and thats why its accommodated to your level. So on the other hand its actually the special effects and damages and benefits, the things that make weapons legendary, or crap or whatever, and of course, because of that these dont scale with your level. Combinations being very important too.
Think of this as in the weapon balance of battlefield or FPSs, you have weapons that excel in different attributes, such as a shotgun in damage vs an F2000 in fire rate and range, and while these guns are great at their jobs, making them each valuable for different things, what do players always do because of how dynamic FPS games are? try to find the most balanced weapon, the one gun that has such a balanced combination of good enough stats, that will be versatile and flexible without sacrificing too much in any aspect. These guns eventually are called OP for that reason.
Now my point with this analogy, is that this is very certainly how things will work in TW3: weapons will progress according to their overall value and practical uses. So a crap sword will just have damage, which any other swrod will give you too, but a legendary sword might have 10 fire damage too, and an even more legendary sword might have 10 fire damage and 10 vitality regeneration or whatever.
This makes it easy to make weapons both balanced and always valuable, there's no such thing as "weapon ruining", because you can find a legendary sword early on, and no matter if its damage is reduced, its other stats will remain to make it legendary as it should be, and nothing can ruin that.
The swords getting ruined argument would only work if swords would truly lose what makes them great or unique, and so far everything said by CDPR points to the opposite. In fact, lets be honest, most swords in TW2 never were special because they made incredible high damage compared to others, they were special because of the special effects and so on, most of them had very comparable damage and the decision of which to keep and improve ultimately depended on its "bonus" features,and in TW3 it will be like this once again.