Earlier quoted context omitted.
It's not so much that EC2 Classic offered any features that were difficult to live without. It's just that migraine away from it means migrating, period. You need to move all of your systems, including any data stored on those systems (in instance store or EBS), to effectively a new data center. Migrating a live production environment can be a pain and/or cause downtime.
It's also worth remembering that infrastructure stuck on EC2 Classic would have been built so long ago it may predate modern cloud tooling and even modern best practices around reproducibility and CI/CD. (EC2 user since 2010)
Exactly - and with fewer managed services back then they’d also be more likely to have hand-rolled servers doing things which you’d now try to hand off to a managed service. I remember entire servers running small tasks which you’d now have, say, CloudFront or an ALB invoking Lambdas or at least sending it to a container.