Eglah arufah is NOT a mitzvah you are able to be mekayaim, so you wouldn’t be considered to be mevatel the mitzvah.
But… by simply wearing an arba kanfos beged, you ARE able to be mekayaim the mitzvah of tzitzis, SO – if you don’t do it when you CAN, then you ARE being mevatel a mitzva a’say.
As I said above, the vast majority of poskim feel that lack of techailes is not me’akaiv fulfilling this mitzvah.