If you are playing only Vanilla (the base game) you are correct that if you are primarily in Blob, scrap out Brain World from the trade row. Essentially, for Vanilla it is Blue -> Green/Yellow -> Red -> Blue
So a high amount of life gain in blue can outlast Green/Yellow damage focus. Generally if you can get heavy aggro early, Red cannot scrap down fast enough, and in Vanilla, red damage is mostly rather small. But blue is slow, so red can scrap down to 5-6 damage cards and take down blue.
Green is much stronger than Yellow, and Red is actually dominant in that format. Scrap is king in Vanilla.
As you add expansions though, change the thinking to:
Mitigation -> Aggro -> Deck Size
Mitigation can be life gain, but it can also be a heavy base strategy. It’s anything that lowers your opponents effective avg per turn damage against you for one of their decks.
Aggro is aggro, damage.
Deck Size is scrap, draw and cycle. All of these things can make your deck smaller, or seem smaller.
In my opinion, of the three base sets, Vanilla is about scrap. Colony Wars is about Damage. Frontiers is about speed/deck size.