First, welcome to FT. However, I wholehartedly disagree with your expiry argument.
If you're biggest gripe with the new program is that miles expiry happens with 18 month of inactivity, I don't have much sympathy. These programs are really not going to be rewarding for one-time flyers, or those who decide to fly UA/a partner every few years anyway. It's meant as a marketing program and to help generate loyalty. If you aren't giving them, or partners, any business within 18 months, its fairly safe to assume you're not very loyal. Why should they give someone rewards if they are not really giving them much business.
It's not even that hard to have activity within 18 months. One of the restaurants you go to every so often probably is part of MP dining. And the mall offers countless opportunities. Buy something from one of the retailers online (you probably already shop at a few of them, and don't realize it). Even if you don't, go there, find iTunes and spend a couple of bucks there, and your miles are safe for another 18 months. You don't even really have to spend any money at all anymore. Not sure about since the changeover, but you used to be able to use miles for a newspaper/magazine subscription, or could donate miles to certain charities. Or...sign up for that TopGuest program and check in through Facebook or Foursquare the next time you are at an airport. Cost to you to extend your account for 18 months: nothing.
There have been some devaluations in the program recently, and I can understand many (though not all) of the complaints that have been put forth in this forum. Yours is one that I can not understand.
On the other hand, I also don't see how you could be upset about both complaints in your post. Are you having a hard time determining how to have MP activity 18 months, or are you a 75,000 - 100,000 mile flyer upset that you are not in the top tier anymore. I may not be an analyst for MP, but I see these as two different people.
With the merger, OnePass no longer exists. You don't have to like it. Take it to court if you wish, but it will be a waste of time and money.