You do not need that information in order to run effective online ads. You've just been conditioned to believe it's necessary by DoubleClick, Google, et al.
Without pinpoint targeting, more irrelevant ads will be shown. That reduces value.
Without lots of invasive tech, you can’t avoid fraud or track conversions. This also reduces value.
This isn’t an unfortunate side effect of GDPR and adblockers.
Of course what you can no longer do (without user consent, because if the user give you explicit consent you can) is to provide ADS targeted to a specific user based on tracking data. But you can assume that if a user visits a page that has a certain content it will be interested in certain type of ads, this is possible because you are not targeting the specific user based on data you collected on the user, but all the user that look at that content will see the same ads (that are pertinent).
Is a big loss? Yes, to Meta and Google, to company doing advertisement not, because this kind of advertisement is not that effective, and to users even more not because tracking is bad for their privacy in many ways.
Before one says "but if you don't track the user you cannot know how many user did see the AD and you cannot target the AD to the user preferences so no one will buy them", because nobody buys ADs on TV, radio and newspapers where you have the same one directional communication channel? Come on, they are only excuses to collect more user data...