I'm a day-one Destiny Alpha player with over 2400 hrs logged. I do a weekly podcast about Destiny and am involved in the community. I couldn't agree with the OP's statements more. They hit at the core of Destiny's issues. Destiny has developed a pattern of putting out the best gear in the beginning and has slowly made it all worse and worse as time has gone on. While they have fixed a lot, the gear and reward systems have suffered immeasurably.
A lot of people have said something along the lines of, "You've already played 40+, just play something different." That is missing the point entirely.
The original game contained great replayability, mainly due to the hunt for gear with friends and the surrounding community. Part of the end game gear hunt is mainly to prepare for the next DLC and level increase. With each new release we would race to complete raids and find secrets.
Now though, it seems that most new gear is consistently, very clearly substandard to the gear we have previously experienced and the new systems, economy, and rewards all seem to work against the ways that regular Destiny players want to have fun.
More of a concern than "I'm not having fun still putting in more hours each week on a $40 release that I've already put in 40+ hours on" is how Destiny continually changes the way it functions and what to expect in the future. You work toward goals that precident has set only to have that effort mean nothing when systems are so drastically changed. The only thing consistent about Destiny, right now, is its inconsistency.
There is nothing wrong with a game evolving and improving it's systems but too much inconsistency or under valuing of player's time or efforts by literally taking their rewards away from them or by removing the promised or implied effectiveness of what was earned, is a disheartening experience to say the least.
It wouldn't be as problematic if the longevity of the game and persistence of your original character through this long journey weren't such a selling point. Bungie has said that
your progress will 100% carry over to the sequels as well as
said things like, "there will be people operating our data operations center 24 hours a day, 7 days a week - they'll never leave - for the next 10 years." and "there will be something new to do every single day."
All of that said, I love Destiny, made great friends, shared awesome experiences and that is why I'm so concerned with the direction Destiny is headed and so let down by its current systems. The biggest problem is that they gave us many of the things that we wanted, we got to experience that, and they have since been taking it all away bit by bit. It really pulls the rug out from under the fun.
tl;dr: I agree with th OP. Destiny started out giving us what we wanted and has been slowly taking it away ever since. Every step forward leads to two steps back and for dedicated players it is becoming frustrating.