Your suggestion is to change the default on the checkbox to "f". Well, it is hard to make a choice, which fits best to everybody's needs. The situation on personal devices (e.g. mobile phones) is as well different as in other cases.
For the time being, i've added a parameter PersistentLoginDefault to acs-subsite that lets an admin decide the default value, without having to hack the login.tcl file. The parameter setting defaults to the current behavior of OpenACS, so people which are updating and do not want to make changes see no difference.
[1] http://fisheye.openacs.org/changelog/OpenACS?cs=oacs-5-10%3Agustafn%3A20190224075820