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The increasing incidence rate is quite interesting.
A common denominator for incidence can often be region, historically.
So statistically the further from the equator the greater the chance of getting MS, i.e., it is surmised that less exposure to sunshine is relevant.
Less exposure to sunshine correlates to statistically lower levels of vitamin D.
Vitamin D has many roles in the human body, but a prominent role is stimulating an efficient immune system.
So, my own thoughts lead me to consider the way life is changing, particularly in modern societies where we discourage exposure to the Sun, and encourage Sun blockers when we are exposed.
This limits our Vitamin D levels except for through diet.
Unfortunately, natural foods with high levels of vitamin D aren't often in our modern diets.
So it becomes interesting to consider how we interact with sunshine and exposure to it, and/or how we monitor or stimulate our Vit D in our bodies, for various reasons. In this case relative to MS.
Personally, I take 10000 iu of vitamin D per day and have done so for about 10 years, and that gives me much higher D levels than the majority of people in Melbourne.
Although that can be good for a variety of reasons, the most prominent is the Immune System, with other areas like bone health and of course its greatly anti-inflammatory.
And it is suggested all disease has associations with inflammation.

So the incidence of increasing MS levels is interesting when considered in the above context.
Unfortunately at this point, people who are getting MS are often in their 20-40 age group, which is not generally an age where people are monitoring their vitamin D, but 20-40, and younger are ages where people are highly encouraged to avoid the Sun.
It could be suggested that a diet rich in vitamin D products would benefit people from a young age, especially girls due to the 3:1 ratio of contraction.

Anyway, just a chat/thoughts in regard to you highlighting the incidence rate BCO