This is, of course, quite right: no DM is ever going to think* of everything, and any expectation that she will is doomed to failure.But only the ones that the DM thinks of. For instance, going back to the tea house. The mayor might be there, as might the warlock's patron waiting to give him a task, or the wizard's master, or... There are waaaaay too many possibilities for the DM to be able to think of, let alone spend hours rolling the thousands or millions of them to see if any of the really long shots happen.
There is no potential for this method to mirror real life, but that's okay. It can approximate things and give the game a similar feel to real life. If the DM only thinks of a few possibilities and one of them hits, say the warlock's patron waiting at a table for the warlock to get there, that's enough to give it the real life feel.
* - be it during prep or on the fly.
As long as a DM can think of enough things, however, to keep the game going and provide within the setting some interesting and reasonable options, choices, consistency, and consequences then all is probably good.