Your question made me smile precisely because it can be argued either way. Therefore, I will share my opinion. I agree fully that this novel is all about "stifled" passion and "packed" regrets; however, what I don't agree with is that it "offers us nothing more than" those things.
First, let's explore the "stifled" passion of Archer. At the beginning of the novel, Archer has "properly" chosen May as his future wife. She is perfect: gentle, ladylike, and upper class. May will make a wonderful mother. However, May's passivity does not incite any passion within Archer. Archer, in fact, feels no passion at all until Ellen arrives on the scene. Ellen is a woman who bleeds mystery. She has questionable things said about both her past (in a wild manner of living) and her present (her pending divorce & her acquiring a new apartment). These things all scream both mystery and passion for Archer.
I couldn't have spoken like this yesterday, because when we've been apart, and I'm looking forward to seeing you, every thought is burnt up in a great flame. But then you come; and you're so much more than I remembered, and what I want of you is so much more than an hour or two every now and then, with wastes of thirsty waiting between, that I can sit perfectly still beside you, like this, with that other vision in my mind, just quietly trusting it to come true.
Can any quote be a better examination of Archer's pent-up passion?!? I submit that there can not! Further, Ellen leads him on to the point where she says they can always continue a relationship, but can never actually make love. The ultimate "stifled" passion!
Next, we can talk lots about "packed" regrets on the part of both Archer and Ellen. The latter has a bit less "packed" regrets mostly because Ellen doesn't care quite as much about what society thinks about her. However, the fact that Ellen (in her time period) is considering a divorce, shows that she does, in fact, regret her marriage. Of course, most of the "packed" regrets revolve around Archer. Archer regrets asking May to marry him. Archer regrets not meeting Ellen first. Archer regrets that he lives in a society that doesn't allow a relationship with women of mystery such as Ellen. He regrets his society as a whole!
The real loneliness is living among all these kind people who only ask one to pretend!
Archer is constantly moaning over all of these things and the book focuses often on his romanticism. Being a romantic, Archer knows that seeing Ellen (even after he is a widower) would incite the same wild passion as his youth. He knows that:
“Each time you happen to me all over again.”
Therefore, he chooses to remain sadly outside, ostracized to a park bench as he laments the past and, yet, proud of himself for choosing a stable future.
The book, however, doesn't offer us "nothing more than" those two things! Just because passion is "stifled" doesn't make it any less real! In fact, hidden and stifled passion can be even more intense than that which is out in the open! There is a certain draw to the characters when Archer is, quite literally, unable to keep away from Ellen. Those feelings create suspense for the reader and intensify the literary quality of the work. Further, the "packed" regrets of which the novel is full end up proving both the positive aspects of living a life within society's grasp as well as the negative aspects of living a life within society's grasp. This holds true even today and, therefore, offers us MUCH more than just the "stifled" passion and "packed" regrets. Further, as a result, Wharton allows her reader to decide whether Archer chose the "right" or "wrong" path due to conformity and lack of passion.
[Archer's] whole future seemed suddenly to be unrolled before him; and passing down its endless emptiness he saw the dwindling figure of a man to whom nothing was ever to happen.