Then I learned because they’re saving it all browser-side I had to rebuild the whole menu whenever I first used a new browser or computer? Whaaaat? Of all people, AWS console users have to be highly likely to be using multiple devices/browsers. Having to recreate your own prefs at each new environment is nuts.

While security and UX are oftentimes in tension, in this case they don't have to be. It would not be that hard to be signed into multiple accounts and allow you to switch seamlessly between them (allow the tagging of each account, such that you can say, effectively, "show me dev us-east-1" vs "show me us-east-1" vs "show me dev", slicing and dicing between accounts that way). At that point, separating infra across accounts becomes semantically meaningful, and you can slice/dice in whatever way seems best (so you could have a full account for a single service, sure. Or an environment. Or a region. Or a combination of those, only service-Foo in us-east-1 for dev. Whatever level of granularity you want; trading off instead between the security of isolation with the convenience of colocation, which should be the actual UX cost; infra in the us-east-1 account has a harder time communicating with the infra in the us-west-1 account).

You can set budgets by project and easily allocate costs and address accountability issues across teams or products.

The value depends on how you operate.