I would state that the basic premise that there is controversy on the value of fiber is .... not accurate. It is well received and virtually uniformly promoted as a health driver. So rather than address the small numbers of "authorities" that remain in doubt, maybe it is better to explain WHY it is helpful, and where are the nuances to be concerned about.

1) FIBRE is used to describe polysaccharides that the human gut cannot digest but yet can be utilized by our gut flora (microbes). They metabolize these polysaccharides for their metabolic needs, and some of the end points of this metabolism are helpful to us (more on that below).
2) Prebiotics are also fibers, but they selectively feed, or are utilized by friendly bacteria. Fiber per se (as noted above) feed or are utilized by ANY BACTERIA, good or bad. Succinctly not all fibers are prebiotics but all prebiotics are fibers.
3) About half of the energy sued by the colonic mucosal lining comes from the short chain fatty acids (SCFAs)generated by bacteria in the gut. So they are important energy sources as opposed to ONLY glucose in blood.
4) These same metabolites (SCFA) also affect which genes are turned on or off. This is via the science of epigenetics, which is the equivalent of your genetic software. This is important as fiber is linked to a reduced risk for colon cancer and this is largely driven by maintaining a healthy colonic epigenome - the genetic software is not corrupted. We now know this feature extends beyond the gut, fiber is linked to lower breast cancer risk through the same mechanisms.
5) We know that mice raised without a mcirobiome (germ free is the term) have significant health concerns, and this reflects gaps in the events driven by this bacteria, and fiber in these cases is less effective as there are no bacteria to utilize it.

These are the primary features of why a high fiber diet is important. There are the age old considerations like "bulking agent" for stools but let's move on to the latest science for a full perspective.

From a personal experience utilising a high fibre diet for health reasons is a bit of a trial and error process. Too much of the wrong type of fibre can result in an 'over' reaction by the body. As any good nutritionist suggests it is important to have a food diary to see what sort of foods you can tolerate and manage. I think it is possibly best to go for a balanced diet and increase the fibre content slowly to suit mixing both insoluble and soluble content so that your body does not 'over react'.
In answer to your question I think the benefits of a higher fibre diet can be proven, I don't think the research has been fully carried out (as far as I know). Publicity and advice from your GP would help get this message across.

The question appears to be posed as if “fiber diet” is some kind of “new panzcea

...sorry - I got disconnected.
...panacea.
The fact is that it is a fundamental part of a normal, healthy diet that is based on

• not eating more calories that one spends;
• eat a variety of unprocessed food such as complex carbohydrates, proteins, fruit and vegetable, and plenty of water;
• avoid refined sugar and saturated fats.

This way, one gets all the fiber one needs!

For better health it is necessary to eat different types of fiber foods which are  necessary and well-balanced diet that includes delicious high-fiber  fruits, bananas, oranges, apples, mangoes, strawberries, raspberries and vegetables as darker in color include  higher fiber content such as beans, legumes,  breads, grains and nuts.

The following are potential risks or complications of the diet (High fiber diet):

• Gas and flatulence.
• Weight loss diets are not recommended for pregnant or nursing women.
• High fiber diet may prevent the absorption of some nutritional minerals.
• Consultation with a licensed health care provider before starting any diet plan and exercise program is recommended.

Any thoughts?

Many foods contain fiber in general are more natural and unprocessed food are with higher fiber. High-fiber foods help to meet your daily requirements keeping digestive system regular and it is best way to ensure always getting the right amount of fiber which is available in foods like   Black Beans, Lima Beans, Sweet Potatoes. Broccoli. Turnips. Pears and salad makes use of blackberries, lemon, scallions, oatmeal, nuts and fruits as apples, berries and pears.