Look at the Chicago times for the relevant trains in the timetables and you'll see that most of the cut trains misconnected in Chicago. It wasn't the times in Texas that were at issue...it's that if you took the Lone Star you could connect to/from most of the other system trains while I'm not sure you could reliably make anything other than a stray weekend Hiawatha Service from the Inter-American. The trains that got cut seem to have primarily been ones where you couldn't make solid connections for chunks of the 1970s.
To their credit, it looks like towards the end Amtrak was improving on this front (again, look at the Inter-American/Eagle: No small part of that spike in ridership is due to the train's times in CHI getting improved to allow connections)...but it was too little too late for some trains (which presumably didn't have a chance to build up their ridership bases before the ax fell).
Here's a chart of the Chicago connections from a 1979 timetable:
http://www.timetables.org/full.php?group=19790429&item=0033
You can argue ridership for the Lone Star was inflated by the ability to connect to the East and if you switched the schedules then the LS's ridership would go down while the I-A's ridership would go up. But there was a big difference here. The TE now also does benefit from less competition just like the EB without the NCH although ridership is always handicapped by capacity (but then fares go up with fewer options like the LSL right now). I still think having choices on schedule as opposed to two trains chasing each other's tails (SM and SS, LSL and CL) is better for passengers in giving them choices. Back then the BL and LSL were within a half hour going east but westbound the BL arrived into Chicago in the morning while the LSL arrived in the afternoon. Damn the host railroads!
It's interesting the Floridian was perfect in terms of connections from the West Coast, leaving late from Chicago and arriving early into Chicago from Florida. Then again Disney World didn't open until I believe 1971 so it might not have been as popular as it is today. I still can't believe more people from Chicago would want to spend two days traveling to Seattle than two days traveling to Florida. Then again a large portion of the US on the East Coast wouldn't have used the Floridian because they have NEC-Florida trains. You can say how "lousy" or slow a specific train is but I still say a bad train is better than no train. You think I get cranky because I have to transfer to get to Chicago? How about passengers from west of Chicago now having to make TWO transfers? I still hate the fact that to get from Texas to Florida or California to Florida you have to go all the way north to Chicago to go south (of course back in the 70's the SL was only LAX-NOL like it is today).
It's impossible to predict population 40 years from now like it was 40 years ago to predict the population of today. But based on recent population, the Floridian cancellation cost Amtrak two of the largest markets (Louisville and Nashville) that don't have Amtrak now, the National Limited cost Amtrak Columbus, and the Lone Star cost them Wichita and Oklahoma City (they did get OKC back but it only gets them to Ft. Worth.
I think the Texas Eagle split during the 90's was the best solution to give all four of the biggest markets (Dallas, Houston, San Antonio, Austin) a one seat ride to Chicago as opposed to Houston not having one today.
If I had to choose between Floridian or Sunset East coming back (assuming extension of CONO for one seat ride Chicago-Florida), SL East would help out southern travelers going to Florida (although they also have to fix the east-west connections in NOL to make it more pleasant) while Floridian gets Louisville and Nashville back in the mix. I would say if there was a same day connection in NOL I'd favor the SL East. But if you pretty much have to go through Chicago to avoid an overnight stay, I'd say Floridian.
In reality, the National Limited really only helped KCY, STL, and IND get to the East Coast (and Columbus to get anywhere). Columbus is certainly valuable but I wonder how big a deal it is that you can't get from the NEC to KCY or STL. Certainly not being on a Chicago route doomed them. Then, I still say Amtrak f'ed up Columbus (and Cincinnati) on A-Day when they got rid of the Cincinnati Limited. Could you imagine a James Whitcomb Riley-Cincinnati Limited (going through Columbus) today?
Then again, Amtrak should make an attempt to follow population trends and at least try to get service back to high populated areas. The US of 2017 isn't the same as the US of 1977, Amtrak should change with them to better serve the US. Unless they could care less about serving the US. Seriously, Catlettsburg?