In my experience, the kind of person that argues about "arrogant 25 year olds that know everything" is the kind of person that only sees their side of a discussion and refuses to understand the whole context. Maybe OP was in the right, maybe they weren't. But the fact that they are focusing on age and making ad hominem attacks is a red flag in my book.
It's also possible the response was "That's an excellent point! I think we should put that on the backlog. Since this data is already a backup of our DB data, I think we should focus on getting the feature out rather than replicating to GCP."
Those are two plausible conversations. Instead, what we have is "these arrogant 25 year olds that have 1-2 years of experience and know it all." That's a red flag to me.
And this is of course valid reason to ignore basic data preservation approaches.
Myself I am an old fart and I realize that I am too independent / cautious. But I see way too many young programmers who just read sales pitch and honestly believe that once data is on Amazon/Azure/Google it is automatically safe, their apps are automatically scalable, etc. etc.
And again, my point isn't that you never need backups. My point is that it is entirely plausible that at that point in time backups from S3 weren't a priority.
Add object versioning for your bucket (1 click) and mirror/sync your bucket to another bucket (a few more clicks).
Yes, your S3 costs will double, but usually they're peanuts compared to all the other costs, anyway.
Debating it takes longer than configuring it.
I have my family photos on a RAIDed NAS. It took me years to get that setup simply because there were higher priority things in my life. I never once thought "ahh I don't need backups of our data" I just had more important things to do.
Maybe having S3 redundancy wasn't the most important thing to be tackled? Does your company really need that complexity? Are you so big and such an important service that you cannot possibly risk going down or losing data?