That was one of the counterarguments to a common straw man of the time. Changing the interpreter line was a valid route. If you choose to use Bashisms, the counterargument went, you just make that choice explicit; just as people who used Korn/Z/whatever shell idiosyncrasies already had to.

As for efficiency, recall that there were hundreds of /bin/sh scripts in startup, administration, and everyday operation of the system, from all of the /etc/rc.d/ (sub)scripts through things in cron and build systems and package install/deinstall scripts to bunches of commands that were /bin/sh scripts under the covers. So multiply that difference by several orders of magnitude.

Amusingly, this is to a major extent still true. On these operating systems systemd is nowadays parsing .INI files over and over with its own custom .INI file parser, instead of a chosen flavour of /bin/sh parsing shell scripts over and over with whatever (Bison/YACC) parser they were using. They haven't actually gone down the SunOS SMF (and s6) route of compiling things into a machine-readable database. And there are still lots of /bin/sh scripts elsewhere, as service management is only one of several areas.

An interesting alternate history would have been if the Z shell had been /bin/sh on Debian and Ubuntu instead of the Bourne Again shell when this came up, and how much of a difference employing zcompile everywhere would have made.