Not years. This whole "saga" has been going on for 5 months, and the suit against Iron Mountain was only filed on 28 July, so it just took weeks to come to this current arrangement.

EDIT: Toned down the comment. Leaving the rest, though, since I can't delete it with the reply below.

The problem in the case of Synapse was that they said they were doing that stuff, but were lying. (I think. The details of what happened there are apparently still not public. Also their bank seems to have been doing some sketchy things too.)

It does cascade. It's called pass-through deposit insurance, it's codified in 12 CFR § 330.5 and 330.7. It has existed since the founding of the FDIC. Its enabling statute expressly provided that deposit insurance should be calculated based on the beneficial owners of a deposit account, regardless of in whose name the account is. [0]

Common arrangements include: HSAs, HOA accounts, UTMA/UGMA accounts, guardians and conservators, mortgage servicing accounts, escrow and title agents, payroll processors, brokerage cash sweep programs, prepaid cards, and yes, fintechs. [1]

[0] https://www.fdic.gov/notice-proposed-rulemaking-custodial-de...

[1] https://www.fdic.gov/financial-institution-employees-guide-d...