That's not feasible - there is no way to execute a RCT on a sufficiently large scale considering that the hard outcomes, such as death or cardiovascular events, take many years or even decades to manifest.
Cohort studies just like the one we are discussing is the most one can expect in this field.
Fundamentally, this is a public health issue and epidemiology tools are appropriate for the field, notwithstanding that the industry has been attempting to frame the issue as one of personal responsibility (they did the same with smoking).
There is also a subtext to the study:
Although the six foods (fruit, vegetables, nuts, legumes, fish, and whole-fat dairy) that contribute to the PURE score are identified as foods having positive effects on health, this is not the whole story.
The findings can be interpreted without resorting to some, not very well characterized, medicinal properties of certain foods.
The effect could also be explained by the displacement of ultra high processed food and starches, as one can reasonably assume that the six foods are not consumed in addition to an unchanged base consumption of harmful stuff.
This perspective subtly changes the story from one that blames the outcomes on insufficient intake of the good stuff to one of exposure to toxic stuff.
The PURE score approach thus can be looked at as one that diminishes toxicity by reducing exposure to toxic products.
The food industry hates this perspective, as they would much prefer to have the issue framed as an interminable series of poor decisions made by individuals; and they are actively working at keeping the discussion framed as such.
Just look at the firehose of junk nutritional research that is published (often sponsored by the food industry in one way or another): an endless series of spurious correlations and conjectures that always come with the disclaimer that correlation is not causation.
This avalanche of propaganda makes the issue appear incredibly complex, and leaves the average person confused and believing that we are dealing with an intractably difficult problem.
There's nothing further from the truth: don´t eat ultra high processed food and concentrated starches and sugar.