The old app works fine with the new hub. There are some different features, and in particular, each of the two apps has a feature called “smart home monitor” but they are completely independent of each other, they just happen to have the same name. 
You have to use the new app to set up the new hub, but after that you can use the old app and many people do for now.
That said, the fact that Samsung has stated that the old app will be going away eventually does mean that I personally wouldn’t set up a new system today that depended on features of the old app. So I think you made the right choice, but I did just want to mention the options.
SPECIFIC differences between "SmartThings (Samsung Connect)" and "SmartThings Classic"