The history breaking crossover concludes here. Will Tsukasa be useful as a Rider or just a hindrance to the Shinkengers?
The Ayakashi not growing after dying is due to the Rider powers. You know the Rider powers allow it to stay in the human world without going back to the Sanzu River to get more water. So just maybe the Rider powers made it a Rider kaijin. When it became a Rider kaijin, it stopped being an Ayakashi anymore and thus it loses its second life and its water weakness.
But the real reason is this, it's Kamen Rider Decade not Samurai Sentai Shinkenger. Decade can't twiddle with his thumbs while the Shinkengers fight the giant monster by itself. Maybe if it was a true VS thing then I can see a giant battle.
One more thing, Kaizin Ride.