"...What would be the negative effect of having too much AT fluid..?"

Just guessing the transmission was engineered "not" to vent/blow extra fluid to the atmosphere because of environmental concerns...

Also every dealership enjoys selling/installing new or rebuilt transmissions...

They are in the business to rake in the money however they do it...

Pre-syntheticaly engineered trannys just vented/blew out extra fluid if they did not go into self destruct mode first...