Nitpick: Not true. The corporation labels by each city may be removed when the civ switches to state property, but the branches are still present. Flipping the civ out of State property will restore all branches to normal functionality.
QFT.
The only way to remove a Corporation is to buy it out with another, competing Corporation.
(It could be a ridiculous exploit otherwise.)
A related hint here: A buyout costs 3 times as much when founding a competing Corporation. However a buyout in one of your cities of a Foreign Corporate Office costs you no additional gold if under Mercantilism!
If you dont spread corporations to the AI the might become very costly to you.
I.E. Sushi - it adds to city size, city size adds to the costs of corps. If you spread it heavily (and have ressources for it) it will run on a significant net loss, even with the Headquaters beeing Wall Street boosted.
Not exactly.
If a particular city's Corporate Branch is profitable, it's unlikely to become unprofitable later.
Map Size and Difficulty have the greatest affect on Corporate Maintenance, but those are constant.
Civics are perhaps the most influential variable but are controlled by the user and so not unforseen or unnoticed.
Number of resources is the biggest variable, but it's a linear calculation, so if resource 1 is profitable, every resource behind it also will be.
Population is a very small variable which increases base maintenance by only 1/18th per population point. (It takes +18 population points to double Corporate Maintenance in a city.)
For example: If you have 20 instances of Sid's Sushi resources, you'd get +10 food resulting in +5 population. 20 resources in a Standard-sized, Noble game should cost 24 base gold. Each population point only increases that number by
( 24 / 18 = 1.33 gold ), so a population increase of 5 would increase the Corporate Maintenance by
( 5 * 1.33 = 6.67 gold ). I assume the city has a Courthouse, so the actual increase to Corporate Fees is
( 6.67 / 2 = 3.33
).
In that example, the increase in Corporate Fees caused by the population increase could easily be offset by working just one Specialist or Town.
Unspecified in either example are increased Civics fees and

yield from better Trade Routes -- neither of which I'm breaking downhere. The summary of it is that everything offsets in such a way that population increase due to Corporations is not as bad as you made it sound.
Sushi/Mining is very powerful. But to spreading it to a large empire requires a sizeable amount of gold, which you may be struggling to get without running SP.
QFT.
A hint here: The base cost to spread a Corporation is 50

, which does increase with inflation. If you intend to spread your Corporation to the masses, do it sooner rather than later [after inflation has risen] and faster rather than slower.