40 K enemy CS in your realm is indeed something to concern your self off, may it be sea travel or land travel.
Sea travel has a little more of the element of surprise in terms of when the enemy is coming.
But dont forget that sea travel is very very expensive, and still quite a risk even for the attacker.
Besides what an attacker does the defender can do as well, you mention for example the defender to be abroad, that means that as long as the enemy torches your land, you can torch his?
Let us just form opinions like this a bit later on, when we learn more.
Right now we can only make assumptions, and i found yours to be rather extreme right now.
If you are a large realm, with half of your troops doing stuff in the West, a quarter in the east, and a quarter simply not mobilized for combat, with many of the troops deployed ranking in equipment damage and expecting pay, then the counter-attack idea isn't really a viable plan, and odds are the troops can't race to the capital before the enemy reaches it. And by the time they do, they'll have a smaller and battered force. And should the capital be coastal, odds are the attackers will have started a TO to prevent any use of the walls.
Before, the enemies would have needed to enter by the border. In small realms, the forewarning isn't large, but the troops are rarely far away from the capital. In large realms, that gave plenty of forewarning to rush troops to defend. In addition, the enemy could not conduct a takeover of the capital before effectivelly destroying your realm, meaning you were sure to get some use of the walls sooner or later.
The costs and risks of such an operation limit who can effectively launch it, but doesn't make it much less potent for those who can.
Please note that I am not saying that everyone will be able to do massive damage by sea, but rather that some will now be able to do unprecedented damage by surprise.
