A question regarding the offside rule in soccer

Football's rules originally defined the goalkeeper as "the defending player who was closest to their own goal line". Having a player who was specifically the goalkeeper, and held that role regardless of where they actually stood on the field, is a relatively modern concept in the history of football.

Therefore, it made no sense to specify "a non-goalkeeper defending player" instead of "two defending players" because the first was equivalent to the second, and the second is much easier to understand.

It still makes more sense to define offside that way, because it covers situations where the goalkeeper is not one of the rearmost defending players.

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In general, when reading the offside rules of any sport, one must bear in mind the purpose of the rule: forcing attackers to maintain possession in order to successfully score, in a game where it is very easy to advance the ball (or puck, in ice hockey) up the field (or rink).

Using the proposed definition would undermine that purpose, making it easier to attack using strategies that are considered undesirable or unattractive.