This sounds suspiciously like the argument that coal mining should continue so as to preserve coal-miner jobs.

I like this analogy. Why should I be concerned about the collapse of an industry whose dominant effect is to make the world a worse place? Especially if, as in advertising, that effect is the intended outcome, rather than an externality, as in coal production.

Or like the prison guard union defending the drug war.

Eh. I work in advertising as a programmer, and if advertising were banned there would still be lots of other programming work I could do.

(I do think advertising is net positive and prohibiting it would be a bad idea: https://www.jefftk.com/p/effect-of-advertising)

I block ads and I still buy things. The world won't collapse if advertising is curtailed.

Consumer habits will just become more unpredictable- and small companies could compete with big companies that spend billions on marketing.

I don't agree. The more likely outcome would be complete domination of big brands because they own the mindshare (e.g. people will remember Amazon.co.uk, they will not remember hundreds of small businesses too).

Even if you were not directly influenced by advertising, you probably were indirectly through word of mouth.