xPass is only a part of Nexgen. I fully expect that any xPass system will be tiered. The problem is not the value of the money, the problem is that giving value for the money. One of the biggest problems in recent years is the lack of a value justification between the price range of the Deluxe and Moderate resorts. There has to be a reason to upsell. Sure, you are going to have a strong contingent who will always go to the resorts. However, there are times of the year when you have full value and moderate resorts but significant availability in the deluxe. You have big profit margin in the deluxe and want something that allows them to sell the rooms without having to discount. There are plenty of Nexgen features that will be available to everyone. You are already seeing some of these ideas. I see a tiered xPass system as possible. I really think the highest tier will be focused on the Deluxe resorts as the cookie that helps them stop rate discounts to fill the rooms. The moderates will probably have access to a lower tier of xPass. I don't think it will be offered to the Value level resorts.
I tend to agree with your hypothesis...however, one must consider the logistics when it comes to the planning and booking end. Say a guest books a week at the Poly and adds xPass Deluxe to their vacation (or however it works). They get it all planned and make their ride reservations, etc....and then they have to downgrade to a Moderate later. Thus, they'd have to downgrade to xPass Moderate and re-do everything. It's like downgrading Dining Plans, only worse.
And while Disney has already established "classes" of guests (whether anyone wants to admit it or not) by establishing 3 levels of hotel accommodations, they're very careful about not offering amenities that are EXCLUSIVE to guests at those resorts (ignoring the monorail, but it was installed when these were the only resorts). For example, just because the GF has a marina and a spa doesn't mean a Value guest can't go there and use them.
Therefore, I don't predict Disney would tier the xPass system in such a way that you have to stay in a certain class of hotel to get a certain level of xPass. I DO see them doing a tiered system, but making each tier available to all on-property guests.
Because now we have a system where all guests have equal opportunity to hit all attractions. Any threat to that level playing field and people start crying bloody murder... ignoring that's exactly what Disney had for almost 30 years.
People argue against FP and similar systems with arguements like its 'unfair' because not everyone gets the same chance in their eye to ride. Or here, Xpass is 'unfair' because it caters to those who can pay more vs everyone getting the perk, etc.
Instead of looking at the impact (like you mentioned), most people just want to put their foot down over the very notion of having tiers at all... and lets not even start with the 'tickets should be affordable to everyone who wants to go' arguments every August when prices go up. The problem is more pronounced with DL then WDW.. but people think the property should be some communal resource or something.
Disney has done a remarkable job of leveling the playing field, while still offering accommodations and vacation components that work for all "classes". Yes, they offer 3 levels of resort hotels, but that's no different than going downtown and choosing between the Comfort Inn, Hilton or JW Marriott. You pay more, you get more.
However, when it comes to things like tickets and FastPasses, everything is pretty much dead even. FastPass is free, and money doesn't give you an ounce of advantage - just timing.
With xPass, if they're going to maintain the level of "fairness", they'll offer the tiers, but offer each tier to everyone, as I mentioned above. The lowest tier should still come with an amicable amount of perks so that the low budget guests can still benefit.
What they should NOT do is cut down on the number of FastPasses distributed in the parks, as that will take away a perk from non-xPass guests. But I believe this effort of controlling return times will allow them to compute an appropriate number of FastPasses vs xPasses distributed each day, for each attraction, and still maintain reasonable wait times in each queue.