Mugai Ryu, Hokushin Itto Ryu, Toda-Ha Buko Ryu, Daito Ryu, Hakko Ryu, Koden Enshin Ryu, Hyoho Niten Ichi Ryu, Yagyu Shingan Ryu, Shidare Yanagi Ryu, Tendo Ryu, Kurama Ryu....
I could go on but I believe I've made my point. There is enough precedent in the Japanese martial arts community to warrant the use of Soke to designate the inheriting headmaster of a system. We can further argue as to whether or not is it appropriate for arts outside of Japan to use the title, but still the historical precedent is set. To quote an article on Koryu.com from William Bodiford:
Even more concrete evidence is the following screenshot from the website of the International Martial Arts Federation (Kokusai Budoin), an official organization overseen by the Japanese government:
Another thing I'd like to point out is that at the end of the description of the first three titles above (Renshi, Kyoshi, Hanshi) is where it says "similar to the title" before listing an academic degree. This has often been misinterpreted by those same instructors as such degree is required to obtain these titles, when clearly that's not what is written in the official policy. It simply states that being awarded such title would be similar in stature.
Now, while it can be argued that they are the most legitimate and universal, the Kokusai Budoin is not the only source of shogo titles in Japan. Individual ryuha and non-government recognized organizations can and do issue them as well, yet just like kyu/dan rank they are generally only valid within that particular group. Shihan, for example, is one title that seems to be all over the place ranging from simply the head of a dojo regardless of rank, up to and including the single most senior instructor of an organization (such as in Shodokan Aikido). My shihan title, for example, comes from the All Japan Budo Association, where an instructor must hold a minimum of 6th Dan to be eligible. More information on the AJBA can be found here.