Well i forgot to mention that without maintenence the dyke will collapse.I agree with all points you mentioned except the one in bold , Dykes though costly is a one time effort which will help preserve vast amount of land , If indeed the base will be washed away the mangrove roots can be uprooted as well , the reason mangroves are holding on is that they are deeply rooted , then build the dykes deep and strong too . build the dykes slow and steady even if it take 10 years to make it , the results will surely help
The thing about mangrove is that its a living thing that has adapted to coastal region, sediments in the water cant break it. Even if it damages the plant, it can grow back.
Mangroves are deeply rooted, this is the reason why the land wont get eroded away as it is held by the roots. Thus the soil does not get washed away. However a dyke does not hold the soil together. If the soil gets washed away the base will be exposed