My evidence is the complete lack of any other widely popular games which only use half their content 95%of the time without having recreated or rebranded themselves with a new version. Perhaps you can think of one or two Vorpal but I can't. At the very least they are extremely rare. As evidenced by X Wing the proportion of game elements which need to be relevant is directly related to the popularity of the game or of it subject premise. This is not warranted it is mandated by consumers who don't enjoy paying for things they only use half of and have much less patience for it if they don't have a Ionic brand (Star Wars) behind it.
Although they are no longer with us I would simply point to the WotC mini games as GREAT examples where over 50% of the content wasn't used 95% of the time. I guess if you want to say "content" was anything in a booster box then the number probably changes but if that is the measure than if you buy a Defender for the Predator upgrade you're still using the 'content' of that package.
DDM has a some minor corrections about three sets in but when it actually made real changes the game pretty much died to me and I'm sure to many others. There may have been updates to old minis in DDM 2.0 but the damage was done.
You refute my point by making it for me? They are no longer with us being the key phrase. I'm not suggesting X Wing is doomed or that the unused content is beyond redemption just that it's a millstone and a challenge FFG. Its not what they want and it's not what the players want. I do see improvement over time with this issue but it does concern me.
I'd say that 80-90% of Magic:The Gathering cards aren't used 90% of the time and that games is more popular than ever, competitive, and 20 years old.
Please try not to be daft. For one MTG is essentially on version, what, 10.0 or 11.0. They release new sets and ban older cards from competitive play. When those versions were in play and legal those percentages you gave were reversed. Only the intentional cycling from away from older versions has made the older versions unused. Maybe you'd like it if X Wing released all new ships in new colors and maybe a few new pilot abilities then told you the old ones could no longer be used? In spite of the churn in versions of MTG many of the older versions of the cards are still used in casual play. it's a false comparison.
Just because you apparently know nothing but the smallest amount about MTG doesn't mean it's a false comparison.
Out of each set of 150-250 cards, the number that see consistent, competitive play is about 10%. If a set has 25% playability in a competitive setting, it's a very good set. This is an example of precisely the situation you described, a very low percentage of cards per set see real usage, but they are still popular and still very much in business.
While it's true the analogy isn't perfect due to the fact that xwing doesn't have rotation or older formats, you simply asked for a successful game where 90% of the game isn't played 90% of the time. I provided this, but that's apparently not what you wanted?