It's very interesting to me that we have to keep telling people this, but it hasn't become part of the "hive folk knowledge" we all seem to develop. I think DB vendors have been sleeping on an opportunity to encourage better practices.
Replication should be fine, unless you mean active-active in which case I suggest a. not doing that b. using interleaved chunks, or a coordinator node that hands them out.
Business data exposure can be avoided (if it’s actually a problem, and not just a theoretical one) in a variety of ways; two of the most common are:
* Don’t use the id in the slug.
* Have a iid column that’s random and exposed, while keeping the integer as the PK.
If you need unique IDs across tables, then I question your use of an RDBMS, because you aren’t really making use of the relational aspect.
Converting int to bigint is not a big project, I have done more times than I can count just like any database evolution. I have also had customers say "oh, we'll just drop and recreate the table every year because its just some trash data." or "oh wait, we didnt mean to create 100 rows every second every day, that's a bug and costing us a lot of money for something we dont care about."
There's no one size fits all in databases, but most people who don't know better don't need to design a database for scale... because their choices won't work in the long term anyway.