This is somewhat of a strawman argument. Passive sensors ability to detect anything falls off with the square of the distance from the obiect to the sensor. And space is really, really big. So if you aren't emitting a lot of heat you simply won't be detected. Now that depends also on just how big and sensitive your sensor is - if your sensor area was say 1x1 km, sure you'd have more sensitivity deeper into space. At a point you simply don't emit enough heat to be detected at a distance. Just how far that distance is varies based upon multiple factors.
Also, some argue that you can't 'cheat' the 2nd law of thermodynamics and converting heat into useful energy that you could then, potentially, use to either power your ship (not totally... that's a bigtime cheat) or else convert into say an x-ray emission and beam into deep space. We already know RTG's work in deep space from the ones we've built on space probes. Just how much heat energy can be converted into useful electrical energy (and how much you'd still lose as radiant heat) is unknown because a) we've never done it to deliberately attempt to control heat emssions, and b) we don't' have access to all the cool materials and tech of the 52nd century.
However we do know, today, that we can do all kinds of interesting things to spoof sensors. Throughout history we have learned to detect, and then to defeat said detection (or at least to make it so that the data you get back is very degraded and not always useful for targetting - you may know something MAY be out there.. .just not much more than that).
Human ingenuity being what it is, I'm sure that someone, somewhere, will figure out ways around these sorts of things to keep the battle between detection/non-detection going strong.