The rot was there since their earliest games, but people forage them for it because their flagship title, Total War, was new, there was room to expand and there was nothing else like it on the market.
By the time Rome 1 was out you could already see they had no wish to improve upon the idea. They refuse to fix the AI, they refuse to address obvious gameplay issues, they constantly introduce new mechanics in one game only to forget they exist by the next.
The same glaring problems that existed in Shogun 1 still exist in Warhammer 2, and each new game is more dumbed down and less challenging, and their games were already a joke to begin with. There hasn't been a single mechanic in place to prevent you from blobbing into an unstoppable juggernaut, no war exhausting score, even the population dwindling (if you somehow fuck up too many battles against retarded AI) was removed, no penalty for taking on too many provinces at once, etc.