Castle Sieges

[Kagemusha]

Sieges were common in sengoku jidai Japan, but not siege battles. Castles were only sometimes stormed by the enemy troops. More often the defender retreated, sallied out or waited for reeinforcements. Off course the lack of siege weapons played an important role, because castles were easy to defend, without them.
Decisive battle were a daimyo used all his troops were the least common form of combat until Oda Nobunga took over and the battles became a new scale. Still many battles were rather skirmishing fights with lots of duels and not meant to cause huge losses like for example the battles between the takeda and uesugi.

I think you are mixing two things here.The many battles of Kawanakajima werent about honourable duels, but even adversaries probing each other.Guess what would have happen to Takeda with decesive loss in Northern Shinano? Invasion of Hojo, Tokugawa, Saito and Anegakoji.For Uesugi, Invasion of Hojo, Ashina, Mogami, Hatakeyama, Shiina and Jinbo. Reality wasnt a totalwar game,where you could constantly put everything for a individual card.

Oda Nobunaga was anomality. After he decapitated Imagawa in Okehazama.He had his eastern border secured by alliance with Matsudaira/ Tokugawa, which allowed him to completely focus to West and North. Its same kind of issue that happened in Kyushu, when 300 Shimazu warriors destroyed the 3000 strong Ito army that created a power vvacuum in Kyushu, that allowed Shimazu to grow as an coutnry wide important clan.