I am fairly confident that in most areas of the Western world the practice of reclining on a couch while eating, haseiba, has long since fallen by the wayside. It is culturally no longer a sign of freedom, luxury, social standing, derech cheirus, or anything else. The Ra'avya's conclusion that therefore we can dispense with the obligation of haseiba at first glance makes a lot of sense. Why don't most poskim agree?
I think the simple answer is that most poskim assume that once enacted, the takanah of haseiba does not simply cease to apply just because the cultural norms that may have given rise to the takanah have changed. It would require a formal repeal of Beis Din to change the law. However, Rav Wahrman in his sefer Oros HaPesach (link) is not satisfied with this answer. If I follow his argument, what he says is that while it makes sense to speak of continuing a behavior even if the reason for its performance is no longer applicable, it does not make sense to speak of a cultural symbol that has lost its symbolism -- that's a contradiction in terms. Haseiba is by definition a symbol of freedom; when it's not, when it's just an empty act of leaning, it ceases to exist -- there is no longer a "metziyus" of such a symbol.
Rav Wahrman suggests a different approach, a classic "two dinim" sevara. He quotes Midrashim that find hints to haseiba in various pesukim in the story of yetzi'as Mitzrayim, e.g. "Vayaseiv Elokim es ha'am derech hamidbar Yam Suf..." These sources point to the fact that in addition to its being a symbol, haseiba is a re-enactment of Bnei Yisrael's behavior as they left Mitzrayim. The poskim who reject the Ra'avya may contend that even if its symbolic meaning is lost, haseiba must still be done as a kiyum of re-enactment.
I don't know if I'm convinced. Let's start with this case: The gemara (Pes 108) says that reclining on the right side is not acceptable because it's dangerous -- food may go down the wrong tube -- and it's not the normal method of reclining. Rashbam explains that it's not normal because a righty would be very uncomfortable trying to eat with his right hand while leaning right. If so, asks the Maharal, what about a lefty -- why is a lefty required to lean on his left side, putting himself in a position of discomfiture, when a righty is exempt from leaning right for that same reason? Why is a lefty not completely patur (he can't lean right, because that would be dangerous; he can't lean left, as that's not derech cheirus for him)?
Without Rav Wahrman's chiddush I would say the answer to the Maharal's question is that Chazal made a takanah based on whatever the convention of the majority of the populace was. Leftys always have this problem of being ignored -- just ask one. School desks are made to accommodate a right handed writer; cars have controls positioned better for right-handed drivers, etc. Rav Wahrman however answers based on his two dinim model: Even if leaning right is not derech cheirus for a lefty, he must still lean to fulfill his obligation of re-enacting Bnei Yisrael's behavior when they left Egypt.
I'm not sure I get it. Why is the behavior of the right-handed people who left Egypt the model all future generations must imitate? Surely there were leftys who left Egypt -- which way did they lean? Why can't a lefty imitate his lefty forefathers? If the answer is that the takanah is modeled after the majority of the populace, I don't see what Rav Wahrman gains in his answer over mine.
There is an even bigger hole in Rav Wahrman's theory when it comes to explaining why women are exempt from haseiba. The Sheiltos (quoted in Rashbam) writes that since women normally do not eat while reclining, their haseiba would not be an expression of derech cheirus and they are therefore pturos. But if reclining is more than a cultural symbol, if it is also a re-enactment of Yetziyas Mitzrayim and therefore divorced from cultural norms, why are women exempt? Rav Wahrman asks this question and attempts to resolve it, but unfortunately I do not yet understand what he means.