Nima, I see no reason why "the performance of the site will decrease
with every single day" due simply to a few thousand extra old users
per department. If the data model is good, even 50,000+ extra
inactive users probably should be trivial for the RDBMS. If
they are not, then something in OpenACS or dotLRN probably needs to be
fixed. Have you in fact seen such performance problems? If
not, why do you think they will occur?