There is a huge difference between a Roman or greek trireme and a 15th century galleass.
To say otherwise is like to say there is no difference between the Monitor and a modern missile cruiser. They're both metal warships. Or that there is no need for a pikemen unit, because ancient era spearmen are the same as pikes. Or tanks and modern armor. etc
In most previous civs, naval units were weak, because they had no real role except for fighting other naval units. In Civ5 they deliberately gave naval units a role by making them able to interact with land units. This was a fantastic design decision, it was really good for gameplay.
So, currently triremes do well in an age of archers and spearmen and need to be careful once catapults show up, but they are massively outclassed when crossbows and trebuchets come around. Getting one-shotted is not fun.
This is why there is a gameplay and realism case for an intermediate vessel, so that investing in triremes is not a total waste of hammers.