The way I was thinking about it was, a disabled syslogger is still part of core and enable-able, therefore the interfaces don't need to change, the system need only be configured to enable traditional log file locations. System builders can set the core option and get what they need, and interested snaps could be adjusted to alert that it needs to be enabled. A mechanism could be provided to allow other snaps to enable core logging setting.
If there is no logger like rsyslog in core, then the interfaces probably do need to change because the log files in question are not provided by core (but are on classic). On core, the log files would instead be provided by another snap (eg, the rsyslog snap), so we'd need to think about if that requires more changes to log-observe or a new interface. System builders could preinstall this snap and snaps could be adjusted to alert that another snap needs to be installed. Depending on where those log files live, those snaps may need code changes for the new location. As things are currently implemented, there is no possibility for a mechanism for snaps to enable traditional logging because there is nothing to enable.