Welp, it's a few days later now, and still no progress. I have all my hotels deleted, so that's not a problem. But still almost zero tourists. What appears to happen, is that all tourists immediately turn "white" upon entering the city on the tourist map.
It was suggested that maybe there was some sort of landmark/zoning mismatch - in other words, it was said that landmarks attract certain wealth level depending on what's in the descriptions (high/med attraction), which I think is complete BS, and that you need to have compatible casinos/commercial zones/etc.
Well, so I changed all my landmarks to medium to compliment my expo centre. Nothing happened. Furthermore, high wealth Sims also visited my landmarks while the "potential tourists per day" dropped dramatically once I got rid of my "high attraction" landmarks.
So it seems without a doubt that your tourism city will cease to function after so many years of game time. Guaranteed. And why the hell do so few people acknowledge this when it is UNDENIABLE and has been going on for SEVERAL MONTHS?!?!? Absolutely incredible. Maybe the majority of players play their cities for 3 years, declare it a success, and move on to something else?