By collapse of digital marketing you mean collapse of social networks?

I cannot say I would miss them, but I doubt it will solve the problem.

Because yes, software engineers would love to work on many interesting things. But they also love being able to buy food and pay rent.

If the ad companies don't pay money anymore, who will replace them?

(I still dream of a world with donation for free services as default, but I am usually not taken serious with this.)

And .. about ad companies and LLMs - I think the madness just started. Once the marketing companies get their product placements directly into the models and the agents sophisticated enough, that you cannot trust anything posted online anymore, it will just destroy anonymous communication, as you cannot trust any anonymous account anymore at all. And will have a hard time finding out who is, who they say they are.