This is poor advise.

It's a good rule of thumb to KISS (keep it short and simple), but it doesn't translate from one short and simplified example to each sentence of a longer text, because sentences aren't wholly individual. Sentences need to work together, in conjunction, because the information content requires a reasonable amount of structural complexity to support the content. Adverbs are very good at that.

I am absoluty not familiar with typed syntax theories or anything really, but I dare say the adverb in the previous sentence modifies at that. At least the parameter is bracketed nicely. Were I to say otherwise that Adverbs are good, it would not be any better now would it? Because the adverb introduces an adverbial clause that modifies the whole preceding text, that is referenced by "at that" in a manner of, err, a fix-point y-combinator!?

On the other hand I am a foreign speaker without good judgement. Consider that good (or happy) itself may be adverbial, Adverbs well support the support.

Good to know: Repition and redundancy can be quite beneficial, and sometimes it's impossible to avoid.

Anyway, kitchen philosophy says that early optimization is the source of all evil (Hoare apud Knuth).

Edit: Another problem of structural support is punctuation

I appreciate your point. I agree that it depends on context. Sometimes I add "very" because I'm trying too hard to "sell" something to my reader. But if I sit back and take a breath... I see that it's unnecessary (and probably counterproductive) to throw so much enthusiasm at them. My writing has generally calmed down (for the better) since I read the blog post that I referenced above.

Why not "He happy". Language complicated. Return to simple. Save energy.

Because explicitly adding implied terms help the reader's brain grip the concepts. Culling all extraneous content makes the result much harder to read, despite being exactly correct.

Make everything as simple as possible, but not simpler.

Why is the writer's role to treat some stereotyped profile of a reader within some optimization function? I have a lot of time to parse essays from an interesting and idiosyncratic writer. I have no time to parse through some 100 word daily blog post from a myopic writer.

I'm more accustomed to the view of writing as expression, though perhaps this is old fashioned. The post-modern engineering view where writing is a crude function seems both totalitarian and impoverished to me. Just look at how every comment in this chajn has explained writing: in the most imperial, imperative way. Thou shalt's and shall-not's, commandment and algorithm.

A lot of that old-fashioned writing is so long because of the per-word rates they got paid to write it and the patience of readers who didn't have much else to do with their time and money. Don't mistake consequences of circumstances for essential qualities.

I don't mean everyone should be Dickens lol. It's fine to be Hemingway, but not because that's the objectively positive way to write.

A piece of writing advice I heard: never use the word "very". There's always a better way of expressing it.

To take your example, don't say "he was very happy", but say something like "he was ecstatic".