Upon reflection… when you give birth to start a thread around here, you really don’t know where it’s going to end up when it develops a life of its own. What I had in mind with the Biden thread was tracking the general direction of his administration, the good, the bad, and the ugly. Sort of an index or table of contents. Accomplishments, blunders, milestones, kudos, rotten tomatoes-- stuff like that.
Introducing Afghanistan into that would take over the thread (I think) and shove off the table other less important, but still important day-to-day things like COVID, evictions (or not), unemployment, the economy, border issues, international relations-- dear God, what a mess that other guy left behind
even before Afghanistan withdrawal.
You mentioned that a thread on LBJ would have to address Viet Nam, but civil rights and the Great Society were also massive issues in his administration. Because of the two-dimensional nature of discussions here, I don’t think it would have been possible to cover any of those well in a single thread on the Johnson administration. BUT it would have worked to have separate threads on “LBJ and Civil Rights,” “LBJ and Viet Nam,” “LBJ and the Great Society,” in order to do those topics justice. Similarly, Nixon might have had separate discussions (threads) on Watergate, China, and maybe his personal racism.
So even though there are already threads on Afghanistan, ISTM that a thread specifically focused on Biden’s approach to that country/situation, past and present, might address your desire for that discussion. 
I don’t claim to be right, and frankly, I was surprised that @What_Exit even wanted to know my wishes. I’ve been slapped on the hand for junior modding in the past, so now I (mostly) keep my opinions to myself on what I think other Dopers should be doing.
Now I need to have some coffee! 