For ad networks: the solve seems to be more disclosure from advertisers of customer PII[1]. All of the recent tracking prevention and cookie restriction measures are disruptive to last-mile analytics, but are far less disruptive to the overall strengths of device and identity graphs. Offline/out-of-band data feeds for click-through tracking don't require passing any identity data, since the click id acts as a key to connect the result to the associated click action. You don't have that for view-through, so instead you pass identity data and that's used to associate the result with the network's device/identity graph and attribute it to any relevant view through action that occurred. But because the advertiser is blind to which conversions/results may be relevant, they have to disclose all results and associated identity data in the process.

Caveat emptor: The above is based on my observations working in digital analytics in general, but my primary focus is in a different area. So there may be nuances or aspects that I'm not cognizant of.

For advertisers: The three main options tend to be either blissfully ignoring (or consciously accepting) the visibility gap, move towards the greater data disclosure of the above solution if you (and your lawyer) are comfortable doing so (which maintains the status quo for visibility while disclosing significantly more data to ad networks), or invest internally in the tech and resources to perform the tracking themselves (which gives the advertiser visibility, but keeps the ad network blind). For option three, you can self-host something like Snowplow[2] and abuse it as a poor-man's ad server for tracking purposes. The Cloudfront implementation model gives you the throughput and latency to allow you to use it as a view-through pixel, and you can then put it on any placements/networks that allow you to use an advertiser-provided tracking pixel.

[1] In hashed form, for what it's worth. The liability of disclosing raw PII is too black and white for comfort, but the security theater of obfuscated PII via hashing hasn't been tested in court well enough to put a dent in that practice.

[2] https://snowplowanalytics.com/