You can A/B test ads pretty easily, and this is quite common. With some degree of statistical certainty, you can tell how one ad performs to another. You don't have control over your SEO results as well - but you can also measure against SEO traffic with a high degree of certainty. All big companies do this. Sure, you're never going to know exactly how many people you advertised to would have organically, eventually…
For sure it's harder for say Gucci to measure incremental cross-platform lift but any good large brand spends big money trying to parse it out
One example way to get some value measurement would be Facebook's powerful tools. Beyond a basic 1:1 audience 50/50 exposure test, Facebook also lets you upload offline sales (or use their pretty effective pixel for real time online sales) and then feed into their system. Using the data your purchase data FB matches to ads delivered and you (with PII but also purchase value and frequency). Then the advertiser can define lookback/conversion definitions to get a measure of incremental lift. E.g. those who clicked ad bought __ right away, those who viewed ad within 1, 7 or 28 days bought __.
For what it's worth the adtech forums I participate on view Uber as incompetent idiots on this issue (which has been reported on like last year) - all they had to do was measure beyond the initial install for say did these installs actually pay to ride...
Install fraud from SSPs and resellers is huge, doing this basic measurement and quality control is the reason Facebook and Apple have huge and growing app install business. Though TBD on Apple fucking over Facebook and keeping the attribution for themselves alone with their new privacy features