The onside kick was a fine decision. Hear me out before screaming at me.
The human mind wants to try to "keep hope alive" by default, and this leads to some irrationality when it comes to how we think about football decisions.
The initial instinct would be to kick deep for most, because it is further away from the mental line of "we now have no possible way to come back." The human mind wants to DELAY that point for as long as we can in a chronological sense. We are wired to avoid the feeling of loss or hopelessness, and we're wired to ignore stacked odds when we have no control over the situation.
When considering the full picture, the actual win probability that we lost by failing to convert onside kick attempt was less than 1%. Pittsburgh, mathematically, was already in the high 90th percentiles in terms of win probability at that point.
The chance to steal a possesion by converting an onside kick attempt is about 5.6% leaguewide this year. That 5.6% chance is absolutely worth gambling a fraction of your infinitesimal chance of winning the game for the upside of suddenly having a REAL shot at winning the game.
Refusing to take a chance on the onside there would've been the type of cowardice that people bemoan when discussing fourth down punt/go decisions. It would've been objectively wrong to kick it deep just to prolong the inevitable as we stared down that barrel of what was, at that point, about a 96% probable loss.