I suspect the original post was triggered by one where the poster was well aware that the fish would outgrow his tank and was planning to take it back to the lfs once he had had it for a while. SO- not a newbie who was unaware, but somebody who followed a deliberate plan of action. I am uncomfortable with this procedure for several reasons:
There is a serious risk that the fish will be kept in the poster's small tank for too long and have its growth stunted, this can happen before it looks like the tank is too small.
The lfs will then have to find another home for your fish- how do you know they can, or that they won't sell it to somebody else with an equally tiny tank? Or flush it- it's still worth their while to keep you happy by making you think they are looking after your fish.
Public aquaria are not always able or willing to take fish that have outgrown their tanks, or if they do, it may cause them problems: my husband and I visited an aquarium the other day and were concerned at some of the territorial/aggression problems we saw in one of their tanks- a tank full of typical ex-lfs fish.
And if you buy that cute plec now, how do you know that your public aquarium will have a suitable space in a year's time? Not something you should count on.
By buying more big fish than we have tanks for, we are encouraging a trade in big fish that is greater than the market of sufficiently equipped fishkeepers will sustain. As soon as the lfs have sold you that plec, they will order more.
On the other hand, there is noone I respect more than the newbie who has made an honest mistake and then does his honest best to rectify it, either by buying a bigger tank or by contacting a local aquarium.
I also have no problems with experienced fishkeepers who breed or grown on big fish for which they know there is a market of suitable fishkeepers. It's when you don't know but tell yourself that it will sort itself out because I really want this fish, that you're on morally dodgy ground IMO.
But to me the crux of the matter is whether the fish has a good home to go after he leaves your tank. If he does, then I don't think you can compare the situation to that of a cat or a dog, who might have got emotionally attached to you. Your catfish won't feel emotionally rejected as long as he's got plenty of clean water and those lovely bits of cucumber keep descending into his tank.