Wonders and luxuries are still global happiness. To say happiness is local is either a misstatement or a testament to how significant building Colosseums in small cities was.
First, as others already have said, luxuries and wonders are not guaranteed. The bigger the map is, the more unlikely it is to get access to luxuries. The higher the difficulty level is, the more unlikely it is to get the wonders.
The point is that the original idea was to have any action (in terms of happiness-influencing) have consequences empire-wide.
To counter this, originally you were allowed to take measures anywhere in your empire.
This concept now has been broken.
Still, there can be actions/incidents causing empire-wide effects, but you are no longer enabled to counter them.
Getting cities from peace treaties now can send your empire into turmoil.
If you puppet them, you don't have the chance to create happiness buildings where needed.
If you occupy them, the turmoil becomes even worth.
In your core cities (which are likely the more productive cities you have) you can't do anything to counter these effects.
If you leave the cities with your enemy, the whole war was invain, as he keeps all the means to recover and fight you again.
The logical conclusion is to take the cities and raze them. That is what I call a game of genocide. And even the razing will severely harm you.
So, what will happen now in games on bigger maps and higher difficulty levels?
Most likely, it will be a combination of just defending your borders to put attrition on your enemy and (depending on how the razing-caused unhappiness will turn out) taking cities and razing them.
To me it looks very much like they tried to "kill" a certain symptom in an isolated manner without having a look at the whole system.
But the main concern is not the nerfs but the indirect nerfs.
