systemd replaces the duct-tape-and-tie-wrap systems that many Linux distros used to run with a unified set of tools, which can be interchanged, but often aren't. You don't have to enable container support, DNS servers, or disk mounts in systemd, but support is there in case you need it. Some distros stopped shipping bespoke DNS servers, others still do, it depends on the distro; it's not like distros are forced to enable the whole set. I don't know any distro that actually does, actually.

If you're the kind of person that wants to deliberately choose what software handles your DNS and what software configures your TTY and which cron daemon you use, then systemd limits your options somewhat. If you want to go in and re-write the init scripts for a system daemon because you have specific requirements, you still can but it adds a level of indirection to the whole thing.

systemd works great for the 95% who can do with a reasonable set of defaults. The remaining 5% will run into annoyances about defaults and obscure configuration, and a small (but extremely loud) subset of those people hate the idea of one set of software taking over so many tasks.

In reality, the option not to use systemd has always been there, but many distros find it much easier to use and most software doesn't bother with the ancient init scripts anymore. You can install openrc, but you're going to need to write most of your service management configuration (often shell scripts) yourself