The upgrade is an very easy process, it will take probably under 1 hour with the actual database size.
You have no idea of the number of customizations Theymos has created. The upgrade would take weeks, if it was even possible. :/
probably most of them are for some background activities for admins, mods and others groups;
the front end is SMF plain and simple with very few standard mods available from SMF library;
and I say that because I know SMF very well, I had and still have few forums on SMF platform
anyway not having the security updates from SMF, is not very healthy; if a stupid bug would to be found in the near future could make a problem;
this is not a forum for kids toys, a lot of people have sensitive informations for sure (in private messages sections most probably, but never know)