I think you simply misunderstood the whole 50/50 rule. If the effort required to reach a goal is X, then for two people to reach that goal the most fair way to deal with it is if they both put in 50% of the work.
Both putting in 100% of the work leads to things being done twice, stepping on each others toes, etc.
You could also view the 50/50 rule as: both parties need to put in the same amount of work X. Whether that is 10 units of work or 100 units of work is irrelevant.
In my opinion, the whole idea of a relationship being ‘work’ is the pitfall of it all. Viewing it as work means you think there is some fair way to divide it. When talking about chores this is true, but chores don’t define a relationship.
A relationship is more like the weather: the clap of a butterfly’s wings can create storms. We don’t know where the storm came from, all we know is we need a way to shelter and deal with it, together. Plus, nobody blames a storm and tries to argue with it, because we know it’s pointless.