Problem being that Infrastructure is made to be very, very compicated because they're selling tons of managed features that have reliance upon each other. I don't know why this idea of having your own devops is suddenly now bad. renting your own bare hardware and managing solutions yourself is still very much a thing and something you have to do if you're using lots of bandwidth. Dropbox for example made their own infrastructure to get off of AWS [1].
I don't think that anyone thinks having your own infrastructure is bad, or if they do, they likely don't have much experience. Rather, I think it can be a nightmare if it's not properly managed, and it's hard to develop the skill to properly manage the process unless you've been burned in the past.
[1] https://techcrunch.com/2017/09/15/why-dropbox-decided-to-dro...