Speaking ASA
While that can certainly be a consideration, that shouldn't be the reason for the award. The runner could have been moving toward home, but would have been dead out without the OBS. Being obstructed is not a free pass to the next base. You should award the base the runner would have attained safely had the OBS not occurred. Returning the runner to 3B should not be discounted if it is obvious to the umpire there wasn't even a remote chance the runner would have scored absent the OBS.
Thank you for the different perspective. Up until the runner had to slow down, it was 50/50 on whether she would be out or safe. At 10u in rec, a decent (not perfect) toss from a catcher to the pitcher at home and the correct tag application (low and in front of the base, not high and over) are not the norm. I can't count how many times I've seen a girl receive a throw on a tag play well ahead of the runner, turn and reach out to tag the runner as if the runner is obligated to come in standing up, only to have the runner slide safely into the base. The toss came in a bit high (head height) and the girl was sliding, which would have required the pitcher to bring the tag down. Absent the obstruction, the runner had a fair chance of beating the tag. I will, however, remember to keep your words in mind when I encounter a similar situation in the future.