The Romulan Star Empire...

I guess so.

They did have some explanations for this on "Star Trek Online," but like everything else, the guy producing this show is ignoring that.

It's like it was made for people who have never seen a ST show before, yet it advertises itself as part of ST canon.

STD did many similar things - but it had the advantage of not having any of the old ST actors - so, you weren't necessarily expecting a continuation of the canon. Here - it looks bizarre.

"Do the writers of this show not even care about previous Star Trek works? Did they not watch TNG, DS9, or VOY at all?"
I would guess not.

They're just cranking out product.

Didn't something similar happen in Star Trek 6? The Klingon civilization came on the verge of collapse when only 1 moon blew up. I remember it being said it was a key energy production facility or something, but you'd think a large space empire wouldn't put all their eggs in one basket like that. That's okay. That was one of my favorite trek movies.

I'm sure the entire Tal Shiar wouldn't have been on those ships that went after the Founders. Even if the Romulan government subsequently decided to round up and imprison (or execute) all of the agency's leaders and disband it, spies are the sort of people who have multiple bolt holes and hidden resources set aside for a rainy day, including false identities. Anyone you don't take by surprise at the beginning and arrest will vanish without a trace. Some along with their inner circles of trusted operatives. Remnants of the organization are likely to survive underground - they may become organized criminals to generate revenue, while still keeping one foot in their old line of work.

Imagine if Beijing were utterly destroyed. Say, by a random asteroid strike. The entire Party leadership gone. Jockeying for power in the various provinces could result in China splitting apart into a number of separate countries. When you have an authoritarian system held together by fear and brute force (which the Romulan Empire certainly was) decapitation of the central authority tends to have that effect.

They haven't really gone into the political situation in what used to be the Empire. Those worlds that were outside the death radius of the supernova would still have to take precautions - like installing some kind of global radiation shielding - to protect their populations. Their ships would've been far too busy at home to send them elsewhere. I think all the evacuees from Romulus itself went to that planet Picard visited. But there are Romulans all over the place now. I'm guessing many of them originated on other worlds of the former Empire. The Federation must at least have agreements with some of those newly independent systems for Romulans to be traveling freely.

Wonder what happened to the Remans?