Regarding the number of countries and continents involved, I think it isn't a bad thing. Imagine how many people only get to see one World Cup in their home country once in a lifetime, if at all. This solution caters to more people getting the opportunity to become witnesses of a major sports event in their home country.
In particular, I didn't think it was possible for Saudi Arabia to host the 2030 World Cup, since the last one was held very close by, in Qatar
2018 - Europe-Asia
2022 - Asia
2026 - North America - Central America
2030 - Europe-Africa (and 3 games in South America)
It's possible that Arabia could bid again for 2034 and could have a chance along with Oceania or South America.
My opinion is that it wasn't a bad decision either, after all the people of South America will have a great opportunity to watch at least one World Cup match without having to travel to Europe/Africa
You actually raised a good point here and I didn't think about that before. But I think there was more to that because initially Saudi Arabia was indeed planning to bid for the World Cup 2030 I think in cooperation with Egypt and Greece or something? But for some reason (not sure if it was political) one of them or even both withdrew and that might also have been the reason that Saudi Arabia stepped back.
As for 2034, the FIFA already announced that the Asian Football Confederation and Oceania Football Confederation will be eligible to bid.