In comments to the final episode, commenter Andy Rose points out that the big revelation is "a little hard to swallow":
While the station may have been financially beneficial to Mrs. Carlson as a write-off, surely it would be much more beneficial as a burgeoning success. (Note how she and Johnny speak about this in a very roundabout way... I presume the writers could never come up with a more precise explanation that would make sense.)
While nothing could make this situation completely plausible, it was actually supposed to be set up a little more than it was. The script for an unproduced episode called "Another Merry Mix-Up" actually contained a scene where Mama Carlson's accountant arrives to look over the books, and is disturbed by the fact that the station is starting to turn a profit.
The script never got beyond a first draft because CBS vetoed it due to the subject matter: it was about Mr. Carlson finding, and smoking, what he thinks is a marijuana joint. (He smokes it to calm his nerves for the meeting with his mother's accountant; after Jennifer tells him that she tried it once -- "In international waters" -- he realizes that his teacher must have lied to him when she said one puff would ruin his life forever.) Though the joint turns out to be oregano, CBS wouldn't allow the script to be produced.
I have a photocopy of the script; I haven't said much about it yet because I'm not yet sure what to say: because it's a first-draft script by a staff writer, it's inevitably not nearly as funny as a finished script. (If it had been produced, the final version wouldn't have been that similar to the script except in terms of scenes and structure.) But I'll do a post about it eventually; meanwhile, here's the dialogue about the station's unexpected profits, which (if the episode had been produced) would have foreshadowed the finale.
The fact that Carlson is unusually prepared and on top of things in this scene is supposed to come from the burst of confidence he got from smoking the "joint."
EMMETT [the accountant]: There's just one problem.
CARLSON: What's that?
EMMETT: That can be worse than losing money in terms of the overall corporate picture at Carlson Industries.
ANDY: It can?
CARLSON: Of course. It gives us triple leverage for a write-off.
ANDY (lost): Uh-huh.
EMMETT: Showing a profit where one is unexpected could have tax repercussions all the way down in the lingerie division.
ANDY: We have a lingerie division?
CARLSON: Oh, boy, do we.
ANDY: I didn't know that.
CARLSON (to Emmett): I'm sorry about unexpected profits. I guess you'll just have to start expecting them.
So that scene, if produced, would have made it a little clearer that WKRP is more valuable to the company as a big write-off than a small profit-maker.