Sea travelling rogues are to islands what normal rogues are to non-islands. Island realms are no worse off than landlocked realms, but simply lost much of the advantage. If we look at the West; Westgard now, but Barca, Terran, Caerwyn, Niselur, and the score of other realms that have been there, they too faced numberless rogues that they could not see coming. Just because sea zones aren't scoutable doesn't make them much different from having vast land borders that just aren't scoutable in a significant manner either, if not only because of the unreasonnable manpower it would take to have nobles in place to scout all potential routes.
Does Westgard scout all of its borders all of the time? No... it's impossible, and even if it was, it's pointless. They come from all the place, all the time. You only really need to know details of how much they are and what they are doing while they are adjacent to your army.
The realms have less nobles, yes, that's true. But the rogue forces are also much smaller. 15k? Seems like that's the average force all over... that's not very threatening. Yea you may lose some regions during the whack-a-mole hunt, but as soon as enough realms lose enough regions, they'll stop spawning. Back in the days, it often got much, much higher, and they didn't start calming down just because realms started losing. They were tweaked many times to make the odds less impossible, but still, overall, people accomplished great things.
If nobles care enough about their realms, they should survive with much more than the colonists first really dreamed of. Regions may be lost, but since rogues now mostly spawn from the largest contiguous rogue region, having small rogue clumps here and there shouldn't be as bad as it was back in the days. Realms would be better off accepting diminished states. Perhaps putting some gold and troops into making their larger states more feasible.
For example, Avernus wanted to relocate. Helping them settle in Itau, Golden Farrow, Paisly, or Candiels could possibly help kill some of these monsters before they even get a chance to swim East, thus allowing you to regain some of your lost regions.