I also think that the advice around Authentication is often misguided. For simple cases it's not that hard to add authentication without a separate service. There is too much scaring the newbies around that topic, when they are probably about as likely to mess up authentication when they try to configure an excessively complex library compared to implementing basic session authentication with the tools their framework provides.
The great big irony here is just that "serverless hosting" is just an extremely silly new name for "shared hosting". The basic economics are the same: instead of paying for a whole server, pay for what you use, then share the remaining resources with other sites.
The new serverless hosts aren't just trying to replicate the "good old days" experience, in some cases they've inherited it. They've also just had the bad luck of hindsight from past shared hosting model problems: insecure FTP is dead (so there's much less of a feeling of "just copy the files over"), allowing direct crontab editing is known to be dangerous in shared hosts so serverless options need their own (often extremely custom) timed kickoff DSLs, hosting static files and application files in the same place is known to cause accidents so there's a greater separation in static hosting and serverless applications, and so forth. A lot of those same complications happened to a lot of the "classic" shared hosts, too. (Which is why many of those entrenched to specialize in known applications over time, cPanel anything you want as long as it is Wordpress or Drupal or one of the other six blog apps the host vetted. The disappearance of "shared hosting" wasn't just that modern development got complicated but that modern shared machine security got harder.)
Someone stripped off that nuance in trying to come up with a concise buzzphrase for code that expects ephemeral access to an arbitrary server.
The only reason we don’t do it anymore in the cases where that would be enough and instead overengineer everything is because others will look us down.
- Some serverless hosting - Some database provider - Some auth provider - Some other service, etc
In the past you just rented a simple VPS or shared hosting, setup all by yourself and have just one provider for everything, nowadays I read that auth it's very hard and you should not do it and instead use some auth provider (Auth0, etc).
I remember the good old days with php, just upload your file and it worked, now they try to replicate that experience but with dozens of providers.