Wouldn't moving the bases back 10 feet add 2 steps? I would expect that the plays where a speedy slapper beats out a ground ball would now be easy outs. And I imagine a stolen base would be rare. However, there would be lots of double plays.
Since we're getting into a hypothetical situation, doesn't this also mean that fielders will be playing further back to protect against the slap or the hit away? This might bring more small ball back to the game as well. I agree that stolen bases may go down but don't forget that your adding distance to the throws from their respective positions so a speedy player still has a chance. I think that the strategies will adapt. The game will change and the circumstances will dictate what a slapper will do. I'm not saying you're wrong. I'm just thinking that with the advent of today's technology in bats, it would be interesting to see how the game evolved under those circumstances.
