> GDPR does not allow you to trade services in return for using your data for ads.
Yes it does. If users consent to it in a lawful way, close to everything is possible under GDPR.
> A website can't deny you service if you opt out of tracking, so EU has decided that business model should not be viable even if both parties agree.
As far as I can tell, this is based on some pessimistic reading of the law, and no clear jurisprudence is available in that area yet. It will likely be available soon, but this isn't as clear cut as you make it seem.
If the EU had decided that this business model should not be viable they could have clearly outlawed personalized ad tracking in its entirety in unambiguous terms. They have not done that at all.
Even so, personal tracking ads are still available for those who consent to it. Many, I expect, will like them. In the worst case (if jurisprudence decides you can't deny services for opt-out - which we will need to away before jumping to conlcusions), it will be a bit harder to gain as much revenue out of it per user because you need to go through the revolutionary step to ask your users if you can track them. You can still show contextual ads just fine though!
But really, consider it ethically. If it turns out people lose revenue because people decide against tracking - as they now need to be explicitly informed and given the decision - how can you really have issue with that?