The problem with making S3 config the default is that it would add friction for new admins. If you're just starting to play around with self-hosting Lemmy then it's 100% certain you have a local disk but not nearly as likely that you've got an S3 bucket ready to go. Making S3 the default would force those new admins to either tweak their configs or figure out how to get an S3 bucket before they could get going. Using local disk for images is fine for smaller instances and self-hosts, but for cost and scaling reasons having an option to switch to S3 is really valuable for larger instance like lemmy.world. Only the top 10-20% of instances are likely to really need to make the switch to S3, so Pictrs defaulting to local disk makes sense I think.