After that, he'll have to rely on flanking (which no intelligent opponent should let him do)
That is probably the strangest assertion I have ever heard.
The reality of the D&D initiative system is that it is
impossible to keep people from flanking you if you engage in melee at all.
When an opponent acts,
both the party Fighter and the Party Rogue will act before that opponent acts again. Maybe it will happen next round, or this round, but it doesn't make any difference because between delay and refocus, the Rogue and Fighter can choose to act in either order - and still go before that opponent acts again.
During their action, both the Fighter and the Rogue can move, and after moving - they can be an opposite sides of the opponent. Mix in tumble and there is absolutely nothing that a meleeing opponent without Uncanny Dodge can do to stop a flank.
And of course, once the opponent is flanked, there is no direction it can take a 5' step that the Fighter and Rogue can't match by simply taking a 5' step in the same direction, delaying action until the other has the opportunity to take the 5' step, and then maintaining the Flank while continuing to attack.
Now, if your assertion is in fact that no intelligent creature is going to get into melee
at all - that's a whole different argument (and one which I kind of agree with - melee is dangerous and should be avoided).
-Frank