Saṅkhārā is a plural word. It is not singular. The Suttanta method says that it is merely cetanā (intention). The Suttanta method cannot account for its plurality against the other mental aggregates. For instance, if you say it is plural because of the cetanā of the six sense doors (Suttanta Method):
‘‘rūpasañcetanā loke… saddasañcetanā loke… gandhasañcetanā loke… rasasañcetanā loke… phoṭṭhabbasañcetanā loke… dhammasañcetanā loke piyarūpaṃ sātarūpaṃ, etthesā taṇhā uppajjamānā uppajjati, ettha nivisamānā nivisati.
Volition for forms in the world… volition for sounds in the world… volition for odors in the world… volition for flavors in the world… volition for tactile objects in the world… volition for mental phenomena in the world is delightful and agreeable; there this craving arises and settles.
You cannot say that because there is the same for the perception aggregate (in the same sutta quote):
‘‘rūpasaññā loke… saddasaññā loke… gandhasaññā loke… rasasaññā loke… phoṭṭhabbasaññā loke… dhammasaññā loke piyarūpaṃ sātarūpaṃ, etthesā taṇhā uppajjamānā uppajjati, ettha nivisamānā nivisati.
Perception of forms in the world… perception of sounds in the world… perception of odors in the world… perception of flavors in the world… perception of tactile objects in the world… perception of mental phenomena in the world is delightful and agreeable; there this craving arises and settles.
So there is a need for the plural to mean the remaining cetasikas excluding the mental factors of the remaining mental aggregates (feeling, perception). This is proof of the Suttas reinforcing the existence and validity of the Abhidhamma.