I got no idea on how modable MTW actually is.
But u're damn right i would prefere a complete redone game in this point.
Hm my idea on the required Nobleman was not that the castle decays completely. Just more like it get's slightly damaged (i'm aware that this is not 100% realistic as there still where mayor of the castles to keep up maintenance) .... it would increase the costs for their upkeep.
My idea for more than one Castle (but increasingly damn expensive from Wood to Stone) was that u build them on strategic Points like Bridges, Streetscrossings etc..as there is nowhere just "one Castle".
I'm not talking of those huge game Citadells, but of small general Castles like they where common. As u could read from my last post, i recomended high costs for stone Castles too - so especially in the beginning u would just be restricted to build wooden Castles.
(The Citadell btw should have been made a Final stage of the City, not of the "Castle" as the Final Stage of it is the newage Fortress or Palace )

And actually if u build those many Castles (for example in the Alps on every Street - what was my original idea in the Strategic importance of more than just one per province) a Enemy might bribe your Commander/Nobleman , and u got a nice Castle with Enemy in your Territory, and a Open passage for Invaders.

And for your Stone Theory ;) Stonewalls decay for sure.
Every Castle, Wall, Fort ect needs upkeep.
Landslides lead to cracks in Walls; lot's of medieval walls didn't have a Basement so simple earth movement, or the wight of the wall itself leads to deformations; Rain erods the mortar, aswell as plaints which get carried up by wind grow inbetween the Stones (thats why ruins get concrete on the copings nowadays)..
And btw a Castle is not only made of Stone ;)
The rooftiles have to be renewed time by time, Beams need to be replaced,
the stables for cattle, horses etc need maintenance ;) the list is pretty long

If u don#t want them to slowly decay away, i'd say the Castles u build should always cost high maintenance-costs

Btw the enemy never stays a chance when u get that mutch money ;) as the AI only is possible to threaten u in the beginning, and from all sides (Played the Holy Roman Empire).