Three kettles of fish/cans of worms to address here...
Regarding Romulans: Gawd, I wish these guys had been able to remain the prominent baddies in TOS. I adore "Balance of Terror" and Mark Lenard's Romulan Commander. As many times as I've seen it, I still mist up. But the uniforms and prosthetics were expensive, and someone dropped the Bird-of-Prey model after that episode. I still liked "The Enterprise Incident", and their explanation that Klingons and Romulans had formed an alliance against the Federation to explain using the (repainted) Klingon ship model for the Romulans. I also adhere to the deleted scene from TWOK that established Saavik is half-Romulan (and will always be Kirstie Alley in my headcanon). I liked seeing where their relationship with the Federation was going through the latter movies (coziness between Caithlin Dar and Saint John Talbot, the Romulan ambassador being part of the conspiracy with Admiral Cartwright and General Chang...). I decidedly did
not like the novels' explanation of "what went wrong" and led to the Tomed Incident referenced in TNG -- including that version of what the Tomed Incident even was. Weak.
So yeah, I have no explanation as to why over the next half century they developed bumpy-forehead syndrome, fired their tailors, and forgot to take the hangers out of their coats. Got some sweet new ships, though. Although I also fill in their fleet with some help from FASA. Repainted, I love their Winged Defender, Bright One, Whitewind, Gallant Wing, and Nova designs.
The former couple represent some nice stablemates to the later Bird-of-Prey, the Gallant Wing makes a good TMP-era evolution of the TOS Bird-of-Prey into more of a cruiser direction, and the Nova makes a brilliant predecessor to the D'Deridex of TNG.
*****
Regarding Klingons: They were brought in to be a cheaper enemy alien race for TOS. Bronze face paint, bubble wrap belt buckles, and some leftover upholstery fabric were all it took. What confuses me is how the Organian enforced peace was handled. "Errand of Mercy" introduced the Klingons, but every subsequent episode with them showed them as adversarial with the
Enterprise -- yet no weapons/gunnery consoles going red-hot. And certainly by the mid-2280s, where Starfleet officer cadets were being taught how to go up against Klingon vessels in combat... Yeah, that ceasefire seems to have vanished at the end of that first episode.
Also, when they finally got a budget with TMP, they developed bumpy-forehead symdrome. I can accept Gene saying that's how he'd always wanted to depict them, but I wish The Powers That Be had gone with his only half joking notion that the Klingons we saw in TOS were the "Southern" Klingons, and these guys were the "Northern" Klingons. Indeed, some of the materials coming out in the late '80s to early '90s involved a power struggle on the Klingon homeworld where the bumpy-foreheads came into ascendency over the smooth-foreheads. I loved seeing Chang in TUC, as that would have been a brilliant way to depict the smooth-foreheads and still have them obviously the same species. I was hoping they'd go with something like that for Kang, Kor, and Koloth on DS9. *sigh* Instead we got the genetic-manipulation/mutation storyline and "we don't discuss it with outsiders". Weak.
*****
Regarding Romulans
and Klingons: I find it fascinating how in TOS Romulans were the never-surrender, honor-and-duty types while the Klingons were sneaky opportunists (cloaking devices notwithstanding)... and then that utterly flopped for TNG-plus. There's a lot implicit in TSFS and TVH. Kruge stole a Romulan Bird-of-Prey (bridge layout is more Romulan than Klingon). But in the next film, the Vulcans had retrofitted it with a Klingon bridge. One can take from this that the technology exchange gave Klingons the small scoutship design, but not the cloak. Also, that the Federation had captured and dismantled enough of the Klingon versions to have a bridge handy, while the Romulan stuff was parted out for study. Then, a bit later, Klaa's Bird-of-Prey
also had a cloak, and was definitely a Klingon ship. Posibility of the Romulan Ambassador's back-door dealing even then? As he almost certainly was the source of the improved cloaking tech used for Chang's Bird-of-Prey...
*****
Regarding Enterprise: You'll notice I've barely mentioned that show. It suffered from the same sort of identiy problems as Voyager, only worse. It either needed to be a show about the early days of the Federation or about the first
Enterprise in Starfleet, but not both. The tech level shown and aliens they were running into... it should have been Robert April on the maiden voyage of NCC-1701 in 2245. Plus, contradictory canon. The
Enterprise has demonstrated a suffixed registry is attached to a particular name. Voyager gave us an NX-01-A named
Dauntless. Thus, the original NX-01 would have had the same name. Yes, it turned out to be a decoy/trap, but given none of our Academy-trained Starfleet crew said "hey, waitaminit -- NX-01 was the
Enterprise! I call shenanigans!", we can presume that Arturis got that detail right. So, given the anachronistic tech and the altered name, I treat Enterprise as a prequel to the JJ-Trek timeline, rather than the Prime Trek timeline. Yes, the finale doesn't work with that supposition, but that's the minority report -- one bit of data versus the entire body of the show to that point. Plus the finale is awful in general, so I ignore it.
*****
tl;dr -- "Bad writing. Next question?"
--Jonah