I think all of us on here have done it at least once. You see an awesome fish, buy it without really even considering what to do with it down the road, and months later you're stuck with a problem. I'll be the first to admit I've done it once or twice... Impulse buys are the most dangerous things in this hobby IMO.
You, on the other hand, know what it is that you're getting into. So I don't believe that "excuse" should apply to you.
If you know that there is no way you yourself can house it long-term, then don't do it.
That being said, I've raised a TSN myself, and can honestly tell you that you'll get a good year out of it in a 75, depending on how big it is when you get it, how much you feed it, and what kind of fish are in the tank with it. One year is plenty of time to come up with a larger tank. So, if you make it your life mission to find a bigger tank the day you buy that fish, and you aren't just BSing yourself about it, then by all means go for it. But if there's even a bit of doubt in the back of your head, then don't do it.
That's just my input though.