I don't think the fact that a team finishes first has any correlation at all with going up, being only related to the referee's judgement of the amount of time that needs to be added at the end of the 90 minutes. If it were so that the team finishing first were automatically promoted it would surely lead to teams not using any of their substitutes, cutting goal celebrations to a minimum, and generally not wasting a single second of playing team.
I should also point out that it is not really possible for one team to finish first because every match involves two teams and they would therefore both finish at the same time and there finish joint first.
The introduction of this rule would of course mean that Manchester United would be relegated every season.