Define "B plot".
TOS did have A and B/C plots in some episodes (often it was the events on a planet vs the events on the ship), although they were usually interconnected.
TOS had them too, from time to time.
Balance of Terror for example, dealt with Stiles and his bigotry as the B-Plot while the newlyweds were a background element/C-Plot.
The Galileo Seven
A plot: Spock and the crew on the planet
B plot: Kirk and the Commissioner on the Enterprise
All Our Yesterdays
A plot: Spock and McCoy and Zarabeth
B plot: whatever was going on with Kirk in the meantime
Who Mourns for Adonais?
A: on the planet
B: on the ship
In addition, TOS would occasionally have scenes of the crew talking to each other or having fun, which didn't have an awful lot to do with the plot - e.g. Uhura singing in Charlie X (which only tied to the main plot at the end of the scene when she starts singing about Charlie and he takes away her voice).
I don't see why should having just one plot be considered a positive thing in itself. If there's not enough meat there for an entire episode, what are you supposed to do, drag it out? And many of the B-plots are fun and provide good character moments and development of relationships between the main characters. Sometimes they're even better than the A plot.
And sometimes it's the B-plot that's more important: DS9 had such episodes, as
By Inferno's Light, where the A-plot is Garak, Bashir, Martok and Worf escaping the Dominion prison, while Cardassia joins the Dominion
in the B-plot!!! Which is also an example of one of those rare episodes where the two plots are wonderfully interconnected.
In the Cards turns the rules upside down - Jake and Nog's effort to get the baseball card as a present for Sisko are the A plot, while the negotiations between Bajor and Dominion are the B plot.
Statistical Probabilities did it again, with Bashir's efforts to help the Jack Pack being the A-plot, while the peace negotiations with the Dominion were the B-plot.
In the 6 episodes of the Dominion occupation arc, A and B plot were both equally important and both about the war/occupation, but one was on the station and the other on the frontlines.
Tacking into the Wind in the Final Chapter - so what's the A plot and what's the B plot? Let's say that the conclusion to the Klingon arc with Worf challenging Gowron and installing Martok is the A plot; the B plot with the Cardassian rebellion at a crossroads, with Damar choosing to back Kira up against Rusot, is no less important and great.
TOS eps have one plot. Usually very Aristotelian, if you're into his thoughts on drama (unity, short time frame decision/danger, etc.)
Aristotelian rules were outdated 500 years ago. If people still thought they it was obligatory to stick to them, we'd have to flush the entire Shakespeare down the toilet.