There is a natural thread drift as other peripheral topics are introduced and "explored" and there is deliberate thread drift to destroy the thread.(weeds wine and walkabouts spring to mind)
If the path the thread is taking does not suit I do not think that gives any individual the right to frustrate it. There is no exterior compulsion to contribute
There are a small number who post who seem to think:
they set the agenda
they control the output
that no one should dispute their opinions
that sole right to censorship should be with them
seek every avenuue to demean opposing views.
They know who they are!

Their behaviour drives many away.