>...outhole bonus ladder that was seldom used prior to 1970 but had a resurgence that never subsided starting with Gottlieb's 1970's 2-player Snow Derby and 4-player Snow Queen?
In my view, the end-of-ball bonus "as we now know it" began with Snow Queen/Derby. The prior implementations weren't as motivating for the player (and hence earnings). The impact on isolating one player's accomplishments from the other players was probably not a conscious decision (IMHO) but more like a change in the priorities for use of the limited number of coils. Drop targets really came on strong, displacing roto targets and most continuous-loop stepper units controlling a variable value. An interesting complication for Jay's idea can be seen in the continuous-loop stepper that controls the "target bonus" in Williams Swinger/Fun Fest. The variable value is displayed in the arc of lights in the lower playfield:
But the outhole bonus is the target bonus rather than the "ladder bonus" built up during play. Also, the loop steps ahead often enough that leaving a higher value for the next player is not a big deal.
As for Bally, they often provided carry-over in the form of trapped balls for multi-ball play. One player can trap a ball, drain the replacement ball, and thus set up the next player to initiate mutli-ball. As far as I know, Nip-It does this. It has an end-of-ball ladder bonus. I don't have the schematic readily available to check the details.
We should also mention that designers did occasionally try to provide isolated carry-overs for each player. Most often, this was done with additional score reels, as in Bally Wiggler:
...but it could be done with separate steppers for each player, as in Gottlieb Scuba:
So the blanket statement that EM multi-players have no intentional carry-over has exceptions. This could be interpreted to indicate that game designers preferred not to give players a way to steal scoring opportunities from other players in a multi-player game. If so, then the many examples Jay cites would be considered a necessary flaw. I think I've heard enough Wayne Neyens talks (and asked him detailed follow-up questions) to know that he, at least, would not view it in such simplistic terms. If anything, designers would like to have incentives for multi-player play.
Designers would be wary of setting up any "community" carry-over features in such a way that the players cooperate to win more free games than they could playing 1-player games. Police Force is one example of that behavior. But allowing one player to collect from the hard work of another might be accepted as valid for friendly (i.e., non-tournament) competition.