The problem with only implementing 5e whenever it is not "in the way" for Larian's ideas is simple: it messes with the balance. Being prone is not losing your turn. It is losing half your speed if you want to get up. It is something you can inflict to get advantage for your melee party members. Same goes for frightened. It is a source to inflict disadvantage, not something you lose your turn over. Action economy is a key part of the balance and these conditions are messing with them in a bad way.
