I have addressed this very argument at least twice previously and do so now for the third and final time. First, the argument can never be - EVER - that you keep a coach because "somebody else might be worse." That line of reasoning means that unless a team has the worst coach available - literally, the absolute worst - then the team should not fire the current coach since they might wind up hiring the ONE guy who is worse.
Second, you determine whether or not to keep a coach based on how well the coach does his job - that's it. Nothing more. Tomlin fails that test. Yesterday was Exhibit #427 in proving that point.
Third, in finding a successor, the Steelers pay millions of dollars per year to their GM, scouts, staff, etc. and it is their job to find someone better. Off the top of my head, some recent hires who teams found among the many candidates:
Sean McVay, Rams: 5 years, 50-23, 3-3 playoffs, SB appearance
Sean McDermott: 5 years, 44-29, 2-3 in playoffs, one AFC Championship appearance
My nominee for the position: Dennis Allen, DC for the Saints. His defenses routinely stuff the running game and rush the passer. He had a horrible stint as the Raiders HC from 2012-2014 beginning when he was 40 years old, but that atrocity was due in substantial part to the Raiders facing salary cap hell and then giving him Terrelle Pryor and Matt McGloin as QB. His highlight as HC was probably beating a grossly superior team in week 3 of the 2012 season, or perhaps beating that same team (once again, despite the astonishing talent deficiency) in 2013.
Trying to remember whom he punk'd in consecutive seasons. God, THAT coach is an embarrassment and cannot possibly still be working in the NFL, amirite??