Good call?

[Dakota]

I disagree with the whole notion that the umpire holds the arm out as long as runner protection is in force and that dropping the arm signals that the runner is no longer protected.

All dropping the arm signifies is that I believe I have held the signal long enough for the players to see that I've got something to say once this play is over.

The judgment I must make on obstruction is "What base would this runner have achieved had there been no obstruction?"

In the general case, how can I possibly make that judgment until the play is complete? Are you saying you decide at the time of the obstruction how this play will finish? You can't. You can make a provisional judgment that you are protecting the runner to the next base, or for two bases or whatever, but how the play actually does unfold can also affect the final judgment.