> A plant could not bite and chew another plant, because, like the fungal cells, the plant cells have abandoned their ancestral animal-like mobility

I would think capabilities like that would be recoverable, if the biological economics worked.

But your point that parasitical plants continuously live off other plants, i.e. they essentially farm them, resolves that. Given victim plants can't run away, their metabolisms are worth far more than any one-time resource extraction.