Of course people have a "right" to recline as far as the seat goes, except during takeoff, landing, and scheduled meal service. The theory is that if each person reclines, the amount of space for each seat stays the same although the shape of the space changes. This made much more sense when long haul flights were very planned and regimented: most often scheduled to run overnight, with set times for meals, a movie, and going to sleep. Now that flights have more diverse scheduling and food and entertainment is often completely at the discretion of each passenger, it works less well. Even if the passenger behind is not uncomfortable reclining, the space available with all seats fully reclined is as inconvenient for, say, holding a baby or working on a laptop as it is for eating a meal. (And of course, slamming the seat all the way back with no warning is not just rude but alarming to the person behind in some situations.)
Totally normal to recline and much better for anyone with back pain or similar issues. Typically more comfortable to recline for people prone to lower back pain; more comfortable to keep the seat upright or recline no more than an inch or two for people prone to upper back or neck pain. Anyway, its just a couple of centimeters at best! There's a huge variation depending on the airline, model of the plane, and route it's designed to fly. Some planes that only do short-haul flights recline slightly or not at all, but (for example) planes on transpacific routes commonly recline 7-8 inches.
Clearly it's pretty common to dislike the idea of everyone reclining all flight and to find it uncomfortable, hence the market for anti-reclining devices and the business decisions of airlines like RyanAir to remove the reclining feature completely. And clearly a lot of other people like to recline, or like to have the option to recline even if some people are inconsiderate in exercising their options. As far as the idea that people should pay money for more seat room if they dislike being reclined upon: I don't think it's frequent flyers saying this; it's increasingly common to be rerouted or placed on standby and have no option over the seat even if you're willing to pay. And that includes some seats (e.g., very last row in economy on many airlines) which don't recline at all even although the seats directly in front recline fully.
I guess it's a hangover from the 50s when maybe people had a degree of respect for their fellow passengers, clearly that's not the case now. I think the fear (much more common and even "normal" now) of confronting a possibly belligerent stranger who may retaliate and make a long flight even more miserable is also a factor that prevents even many reasonable passengers from discussing such matters and having enough information to be considerate/compromise (e.g., warning the person behind that you're putting the seat back, asking the person in front to wait ten minutes as you're feeding your baby, etc.)