In the short term I expect companies like Liveramp (which acts as DMP (Data Management Platform)) to get more business because one theory here is all free apps will start to force users to login (using email/mobile number) which effectively becomes another ID you can track folks with across-apps over time.
I don't want to be stalked online, and I'll raise a glass over their bankruptcy notices.
Also, forcing users to login may also be in breach of GDPR, if you don't have a good reason to need a login, you're unnecessarily collecting personal information.
One obvious example of this is NVidia, you've paid for the card, yet they force you to login to use their apps.
One thing I am curious to see (how it pans out) is if device fingerprinting (IP + Device+ Some other attributes etc. to map you) evolves to solve this problem.
In my experience, it is pretty bad (at best 30% compared to IDFA/Device Id solutions) but I see few companies pitching this as an alternative.