Eh? You *do* have a choice of which init system to run; many people
running Debian are still using sysvinit, myself included. The system
handles this just fine; if you file a bug report via e.g. reportbug, it
will automatically detect which init system you're running and include
that information in the bug report.
What you don't have is a choice *in the installer* of which init system
to *start out with*. The installer will always set up systemd (possible
unusual situations involving preseeding aside); if you want sysvinit
instead, you have to break out of the do-things-for-you friendly install
process and do some package installs + uninstalls by hand.
This makes it harder for people to get a non-systemd init system in
place (particularly people with less technical experience), and harder
to be sure that all traces of systemd-used-as-init-system have really
been removed from the machine.