'Less good' vs 'worse'

Garner's fourth , page 263, reads

Depend typically takes on (or, less good, upon). When a clausal complement follows the verb, to omit the on is a casualism—

Is good here an adverb?

Why not use worse instead of less good?

Solution 1:

Less good means that you go from good to not so good, but still good (we are still in the "sphere" of good).

Worse means that you go from something bad to something that is even more so (therefore, there is no good category in the sentence).

There is plenty of information on that in this EL&U answer, so I won't harp on.

As for the syntax, have no doubt that good here is part of an adjectival phrase and not an adverb. In fact, good is always an adjective. Grammars will tell you so, Cambridge will tell you so, and this answer on ELL will acknowledge it too.

I would argue that less good is an adjectival phrase [quantifier adverb "less" + adjective "good"] which modifies upon (upon is less good than on).

If it had modified a possibly elided noun like choice or use, an article would have been necessary:

Depend typically takes on (or, [a] less good [choice], upon).