This is a case for a better structured web, not for giving up meaningful hierarchies altogether. It shouldn't be possible to register a .co.uk domain unless you are a company that is registered in the UK and can be sued. Ideally companies also shouldn't be allowed to use TLD not designated for them so barclays.awesome etc. is suspicious. A namespace like .bank.uk would be even more secure.
> Subdomains off barclays.co.uk are also not great, because cookies can be shared up to the domain level (intentionally or accidentally),
There is no reason we can't treat barclays.co.uk like a top level domain for some purposes, using a mechanism like the public suffix list.
> But I like cyde.dev better, so I'll be migrating stuff over to it.
So the new TLDs encourage people to migrate, thus changing URLs and frequently breaking them by not bothering with redirects.