AWS used to be not only sane but elegant: every instance had an entirely-arbitrary internal/private IP address, some could optionally have a second public address, and which instances (including external IP addresses) could talk to which other instances was entirely and solely defined by security groups (as well as, of course, any OS-level firewalls that you'd generally disable), which were pretty much just flexible and reusable firewall rules where the concept of a "security group" replaced
entirely the concept of a "subnet", which became an obsolete legacy concept.
They needed to support multiple adapters per instance, which they later added (maybe with a separate security group per adapter, which they might support now but I don't know off-hand); and they also needed hierarchical security group inheritance (the same way traditional subnets can nest into each other), which they didn't add but I guess you can now simulate them (though this sucks and I think is part of the downfall of the elegant stack) using multiple non-hierarchical security groups (which was not supported originally: security groups were permanently fixed in a one-to-one relationship with an instance).
This original elegant cloud-first model of instances and groups made network engineering pleasant for once... even fun! I remember thinking how great it was that all of my arcane physical networking and routing knowledge might soon be obsolete: that I could now think in terms of the abstractions of instances and how they talk to each other, drawing abstract circles around them without having to think about limited address spaces, and that they would assuredly fix the only two shortcomings of the original model...
...but then the network engineers showed up in force and ruined it all. There is simply no good reason for all of this VPC IP-address subnet focused insanity once you go cloud: they are just re-instating all of the frustrating limitations that come up when doing real world network engineering, presumably because they weren't willing to throw away their knowledge and realize all of that stuff is obsolete.
Like, seriously: we want to be able to replicate some enterprise network? That's madness, and it makes it all worse for everyone that this is even a supported goal. This is all virtualized networking: we don't need to be thinking in terms of subnets and gateways, we don't need to be manually configuring our egress... if you have a ton of hubs and routers and have to run cable all over the place, it makes sense, but this is the cloud!
And so now we all actually had to brush off all of that networking knowledge I was happy to give up as Amazon deprecated and fully removed "EC2 Classic" and have forced us all into this VPC insanity; and maybe if you never really tried to grok how AWS worked 15 years ago when it wasn't pretending to be a pile of legacy networking equipment you just shrug and accept that this somehow is all necessary, but it really isn't.