There are other meditation traditions that don't prescribe one specific sitting posture. In the Shambhala tradition, for instance, you can sit cross-legged, but you can also use a chair if that's what works for you. The point of the meditation isn't to torture yourself with a physically uncomfortable postion; on the other hand, this meditation practice (and zazen as well) do bring you into contact with discomfort in the larger sense, and help you develop tools for being present with discomfort rather than trying to avoid it or escape from it or numb it out. It's a very valuable skill.
As you say one does not have to sit in seiza /half lotus/full lotus to practice zazen.If you need to use a chair thats ok.My own feeling is that sitting in a manner which gives you pain,numbness,cramps in your legs and can damage your knees is a bit counter productive.Its a bit like priests in the Middle Ages wearing hair shirts,practicing self flagellation in the mistaken belief that somehow they get closer to
God by exacting pain/discomfort on themselves. Maybe there is a hint of masochism here??Beats me.Please.