Generically: Identity is a very hard space. It's one of those things that sounds really simple but the deeper you go the more edge cases you find.
With an complex systems, changing fundamental designs of the system- while users and other systems are currently using that system- is like trying to upgrade the engines on a plane while in flight. Technically possible, with a lot of planning, but any wrong move or bad assumption and you're going to crash.
I can say for certain that no, it is not something taken seriously.
I (loudly) complained to AWS enterprise support in 2017, 4 years ago, about this issue. Got escalated as high as could possibly be escalated (team leads of IAM calling my cell phone).
4 years later and the issue is unresolved. This is Amazon we’re talking about. There’s very little that can’t be fixed in 4 years.
There is a very large project to fix this. But as you can probably imagine, with a system this large and complex making changes to fundamentals like identities is hard, and done with a lot of care.
All new accounts created today are "decoupled". They are not your Amazon account, and (barring some special internal tools for testing) you cannot make them the same account. But migrating all the existing accounts without causing harm is a messy business.
My own AWS account has the same problem.