You believe all of the OT films are of the same quality?
First, quality is non-existent property when it comes to art (i.e. completely subjective things).
It’s nonexistent in the fact that it’s not something physically tangible but it exists in the mind and that is all that is pertinent for this conversation.
Quality of a product (and its properties) must be measurable, otherwise it is not quality. If it is measurable, it has to have an objective measure. Completely subjective things (like film) do not have an objective measure. There is no objective measure for properties such as story, acting, dialogue, music, sound, etc.
I’m going to have to disagree with you saying that film is completely subjective. Whether you like something is totally different from if it is a good thing. You can like an objectively bad film. I can say that the dialog in the prequels is objectively bad: It’s stale, it doesn’t make sense, etc. Just like with other fields, film has objective qualities that can be measured.