It often seems that beginning advertising is not the first step on a slipperly slope. Not having a plan to avoid advertising is the first slipperly step.
This is due to having so many examples that not having advertising is the first step to having advertising, and that having advertising will be optimized for profit, and frustrate users.
I think the problem is that advertising is one of the few areas where you can scale revenue without the user’s permission. Once you start depending on it, there’s always pressure to beat last quarter’s numbers and it’s easy to tell yourself that users don’t care, and the heat if any arrives years later.