Should mechanophilia and plushophilia imply objectophilia? This implication would be technically true- plushies and vehicles are objects- however, these categories are much, much larger than 'objectophilia' and would make finding stuff that's not vehicles or plushies difficult.
There's also an issue of definition. Mechano/plushophilia still include things like pinups and objects with genitals. Should these terms be restricted to sex acts, or be open to any sexualized subjects?
Mechanophilia also includes anthropomorphic robots that might not fit with what people searching 'objectophilia' are looking for.