Imagine a product, let's call it Oxygen, that every single person buys $10 worth of every month. It has 100% market penetration.
One day, Oxygen Corp decides to take out a $1M ad buy. They reach 1 million people, all of whom then go on to buy $10 of Oxygen, as measured by cross-site conversion tracking. $1M ad buy to move $10M of product - that's a whopping 10.0 ROAS. Must be the most effective ad campaign ever run, right?
From a gut feeling I can say that the customer came from some result of previous advertising (as we're talking about products like sodas not life-essentials like oxygen), but I guess there's no way to know since those previous ads being tv/print ads were not tracked.
If we suddenly stopped all other forms of advertising and only used online ads, in 10-20 years each customer can be tracked exactly to what ad created the first impression about the brand and thus be more accurate (still excepting marketing like word of mouth though)
You can't measure it across multiple advertising platforms very effectively though.
As you said yourself, b) breaks down for multiple platforms. It also breaks down if users have disabled tracking.
They are essentially what one needs to measure incremental ROAS of one's advertising.
Now, there are obvious caveats (online, tracking etc) but they're much much better than what's available for other platforms.
Almost all users who disable tracking also block ads, so you are still correctly measuring the effect of your ad spend.
PS: Of course that works if the difference is indeed small enough in your case so that it CAN be ignored.
I don't want to be offending but this is honestly a very basic point, of course one would only measure the Return On Ad Spend from the user's acquired by said ad and not any others... Advertisers are familiar with all such basic statistics and Google/FB ads give you very easy tools to track any person who clicks on an ad throughout their entire journey.
Edit: I was mistaken as described in the comments below