Gavra, there are rules, but there are exceptions. Part of halachah is knowing which heterim were historically accepted and which were not.
The heter for chodosh, for example, surely would not have been accepted (and likely not offered) if not for the tremendous sha’as had’chak. How to deal with the contemporary situation is fascinating; I find some who feel that since the situation changed, we should therefore be machmir,but others feel that once the heter was accepted for generations, we can continue to be meikil. Both approaches are backed by legitimate talmidei chachomim. I know people who are makpid on yoshon but not C.Y., and people who are makpid on C.Y. but not yoshon. Each think the other’s got it backwards. (Puk chazi mai ama d’bar which is more common.)
The issur of music during Sefirah, OTOH, for whatever reason, has been widely accepted, and I think the acceptance by the tzibbur makes it binding on yechidim, even if the halachic basis for it may seem a bit flimsy to us.