I think that TOS was far more the Kirk, Spock and McCoy show than VOY was the KJ, 7 and EMH show though.
There are only 3 names in the opening credits of TOS because they are the only stars of the show. The others are in the closing credits and were only meant as a supporting cast. It was only due to fans and films that they also got starring roles.
While I don't disagree (it was how the system rolled at the time), I do think that, for example, Jimmy Doohan's agent
really should've been pushing for better billing for his client. Scotty was a very firm number 4 IMO, and probably about as integral to most plots as Bones was.
Actually that's a point. People always talk about 'the big three', but even in TOS I reckon Kirk and Spock are about equal, with McCoy being far less directly a 'star' than either of them. Even after he was promoted to opening credits.
I think for Jimmy, it comes down to budget.
Trek wasn't a big production show originally, I don't think they may have been able to afford a pay bump for him.
To be fair, all the Starfleet personnel got a lot of screen time in scenes that had nothing to do with them just because they were on the bridge or in the away team.
I do think Kes was more popular than Neelix, Kim, Chakotay, she was however the hardest to write for and hardest to market. I think that must have had something to do with the decision to axe her. She couldn't be naturally involved in Starfleet scenes because she wasn't Starfleet, and she couldn't be naturally involved in diplomatic outreach situations because she wasn't experienced. Also due to her age and inexperience you can't easily have her in sexual situations without bordering icky. To use her they had to make stuff for her to do. Whereas Neelix is the 'Outgoing wacky' type so they could just make him do whatever they wanted.
No way was Kes more popular than Chakotay.
I don't care how people feel about his acting, before Seven, Chakotay was presented as the male sex symbol of the show. They died his hair to make him appear more youthful. You really think showing the characters ass in "Tattoo" really need to be a part of the story?

You can't talk J/C sexual tension without him. It held the lady viewers for 7 years.
Neelix was never in the formula because he's the only full latex alien character on the show. His unique look kept him around. Imagine how dull a cast photo is without him, even B'Elanna Klingon features were softened to make her appear more human than alien.(Remember her bushy eyebrows and how she pushed her teeth out slightly when she spoke during the first few eps.?) Every Trek show needs an "Alien" Alien. Neelix was it. Popularity by being unique.
"People" magazine boosted Harry Kim's popularity. The publication is looked at far more than Star Trek.
"Sexist Single Men" issue? Once again, press hyped as a sex symbol for female viewers.
Kes had NOTHING in her corner compared to that. The show needed to get noticed, especially as the launching board for a new network. A sex symbol or unique look will do that. Kes was cute with her Tinkerbell look but it's not new nor exciting. So yes, I agree with your assessment of her not being marketable. She wasn't a sex symbol nor was she going to sell toys, T-Shirts & video games. That's the way the sci-fi market was trending at the time. Who was going to notice Kes when they had Xena to contend with?
At the same time, why waste the actresses time if they didn't feel compelled to write for her? If Jen Lien REALLY wanted to be a actress, isn't it unfair to her to keep her in a contract acting on a show that wasn't going allow her to fully use her skills? Why not let her go, so should could seek out another show or film that would. If Lien wanted too, she could have gone on to a brilliant film career and surpassed every single one of cast of Voyager. No reason why she couldn't have.
Considering how Chakotay, Tuvok, B'Ellanna, Neelix, and Kim all got the shaft. I think TPTB just didn't know what to do with any of those characters; in the situation they were in (lost in the DQ). They were all pretty much empty suits, whose roles could've been filled by any other crewman on VOY. Tom escapes the trap because he's written to be quirky and a jack-of-all/know-it-all character.
I disagree, I thought B'Elanna got great development. She was the only other character besides Seven & the Doc that had a clear defined, consistant and conclusive story arc. I think they real delved into the psyche of her character and Dawson bought such life to her, I found conviction in her. I think the writers were brilliant in their exploration of what it is to be a child of an interracial marriage and ask the question "Who am I?". I found it interesting that most viewers don't pick up on the fact she was bullied as a child for being Klingon, showing there is still racism in the Federation. I though she was the most real hands on character on the show.