If i was making my own GitHub, i would make it possible to put one deploy keys to multiple repos, same as in Gitea. Why that doesn't work is beyond me and is a major pita with no simple work arounds
I don't think app complexity and sophistication grew that much. Most of the problems common apps are solving can be dealt with standard CRUD-like interfaces and old boring tech and it works just fine.
I think what drives this crazy train of overengineered solutions of SPAs and K8s for hosting single static page is deep separation of engineeres from the actual business problems and people they are trying to help. When all you have are tickets in Jira or Trello you don't know why you should do or if they actually benefit someone then it's natural to invent non-existant tech problems which are suddenly interesting to solve. That is natural for curious engineers and builders. Then mix in 1% of big apps and companies which actually do have these tech problems and have to solve them and everybody just wants to be like them and start cargo culting.