No, not less effort, just less likelihood of those people being friends or sharing the same chemistry as the three in question.
The chemistry part is pretty much a happy accident. It's certainly not something that can be forced, or even expected. Shows are put together for maximum ratings (they hope) and they get who they think is good for a role.
That is not true at all. Chemistry between actors is one of the primary things that casting directors look for. It's not an accident at all -- there's a well-defined procedure to it. Actors under consideration for an ensemble cast are given "chemistry reads," tested alongside each other to see which combinations have the best spark between them. That's just as much a basic part of the casting process today as it was half a century ago. There are auditions, readings, screen tests, a whole lengthy process designed to take chance out of the equation as much as possible.
Besides, real-life friendship has little to do with it. This is a job. It's a skill that's developed over years of training, and while it's meant to
look as close to reality as it can, it's not the same thing. People who are good friends in real life may not have any onscreen rapport, and people who can't stand each other in real life can have fantastic onscreen chemistry.
And even if it did matter, again, why would anything be different today? Why would today's actors be less likely to
just happen to be good friends? It's not like Shatner, Nimoy, and Kelley were friends
before TOS. They'd barely met. Shatner and Nimoy had worked together once on
The Man from UNCLE, and they'd had only one scene together in that episode. And remember, all three of them were cast separately -- first Nimoy, then Shatner, then Kelley, even though Roddenberry had been trying to get Kelley onboard since the first pilot. So they weren't cast specifically with their mutual chemistry in mind -- that
was a lucky accident in that case. But with today's ensemble casts, testing specifically for chemistry between actors is a routine part of the casting process. So if anything, you have it backward.
Also, shows have
always been put together for maximum ratings. Why in the world would you think it was ever any different? That's how commercial broadcasting has always worked as far back the days of radio.
