In a society within which increasingly, people respond to reason and evidence factionally, the “dissolve the line” recommendation is a dangerous recommendation. Just try discussing Donald Trump with a Trump hater and you find that regardless of institution of a corporate tax cut which should benefit the average American citizen, Donald Trump is considered a total degenerate. If people will not recognize what is good simply because they belong to another camp, upgrading opinion to fact is recipe for confusion, chaos, and degradation of intellectual thought or intellectual discourse.
I understand what you are getting at in emphasizing importance of assumptions in characterization of opinions, which is, within context of a specific assumption, a statement can be regarded as factual.
Factual, however, is not the same as fact, as in stylized fact.
Factual is not necessarily truth because it always is conditional on context. For illustration, a marriage certificate obtained legally is fact. Regardless, if a man marries two women in the United States, he is a criminal. If a man marries two women, with one marriage initiated in the United States, and the other in Nigeria, or marries two women in Nigeria, he is polygamous. The fact is the same in either case, marriage to two women. The interpretation — criminality vis-a-vis polygamy, which is factual, is function of context.
Regardless of importance of difference between fact and factual, we must assess reasonableness of assumptions. In case of coffee, is it reasonable to value its energy properties over its body relaxation properties? Coffee for the most part lacks body relaxation properties. Black tea possesses some body relaxation properties, hence British tradition of afternoon tea. Take green tea, and you obtain heightened body relaxation properties from preference for tea.
A focus on energy properties of coffee to exclusion of body relaxation properties cannot be characterized as fact based discourse because it abstracts from a property of significant importance, a property which favors its primary competitor — tea. While focus on energy properties of coffee is factual, a choice to ignore its lack of body relaxation properties ensures factual does not translate into fact.
Suppose, however, we put out all of the benefits of each, tea and coffee. Well, some folks will value the energy properties of coffee more. Some other folks like myself will place more value on the body relaxation properties of tea. In presence of facts about each product, there is diversity of preferences or opinions. This as you suggest is the safe route. Dissolution of difference between opinion and fact militates directly, however, against the safe route you propose. The two recommendations conflict with each other because you simultaneously recommend upgrading of factual to fact, an upgrade which without question will create more of intellectual confusion than progress.
While assumptions and context are of significant importance in management of intellectual discourse, they must be stated as such — assumptions and context — not upgraded to facts. This is the only safe course in an increasingly factional intellectual world.