If you really hold there is NO minhag hamakom in EY and only minhag avos, then you get into a big problem. When Beis Din determined Rosh Chodesh based on eidim, chutz l’aretz kept 2 days YT as a safek. ~1500 Hillel HaKatan authored our calendar and since then 2 days YT in CLA is a minhag, in EY the minhag is to keep one day and in CLA 2. If one moves from CLA to EY with intention to remain we say his new minhag hamakom trumps 2000 years of minhag avos and one starts keeping only 1 day. If there is NO minhag hamakom today, as some here have maintained, then all Ashkenazim and most Sefardim who came to EY from other countries should maintain their minhag avos and keep 2 days YT.
Now that we have proven that there IS minhag hamakom in EY even today, it only remains to debate if the few individuals who wear tefilin prevent the 99.9% who do not from obtaining the status of minhag hamakom.