Plenty can do it: USA, England, France, Germany, Spain, Italy, Japan can all do it with little to no new stadiums.
That's hardly many, especially when you consider the rules around hosting in the same continent within two cycles.
Hosts need to have fourteen all-seater stadiums with a minimum capacity of 40,000, and at least seven of those have to already exist. Two of the stadiums need to be at least 60,000 capacity to host the semi-final, and one needs to be at least 80,000 to host the opening game and final. It's not even so much the stadiums, but the training facilities. Hosts need to have 72 training facilities with at least four available per stadium.
Your assessment of the countries that "can do it all with little to no new stadiums" is being very generous.
England has 13 stadiums of over 40,000 capacity, and six of them are in London (and one of them is a rugby stadium). Assuming they wouldn't want to just host half the tournament in London (or if they'd even be allowed to), you're looking at building or seriously upgrading at least four stadiums, if not more when you consider the state of some of them.
Spain only has 10 stadiums of over 40,000 capacity. France only has seven. Italy only has six. I can't speak for the standard of the facilities at these stadiums and would imagine at least some upgrades would be needed to just the existing.
Japan and Germany just about squeak in, but again, I can't speak for the standard of facilities.
There simply aren't that many countries with the capacity to host a 48 team world cup alone. Joint bids are going to be standard going forward, unless you have a massive country (and even then the US went for a joint bid), or a country like Saudi that will just throw loads of money at it.
This also ignores all of the other infrastructure that would be needed (e.g. training facilities, accommodation for teams, officials and fans, transport networks, etc.).
I actually don't know what the deal will be regarding this supposed Saudi bid for 2034. I can only assume they're allowing them to include the yet to be upgraded/built stadiums included in their hosting of the AFC Asian Cup in 2027 as their "existing" stadiums, because they currently only have two that meet the minimum 40,000 criteria.
FIFA have set out their stall now of allowing at least six automatic qualifiers, not even from the same side of the globe. I fully expect a number of daft multi-nation bids going forward, with FIFA eventually expanding the tournament further to allow 64 teams.