Not even close. No matter how porous the foam is, you can't match a spring. But bear in mind this factor can or cannot be relevant depending on where you live. If you live on a dry place, it does not matter; also if you take good care of your mattress (e.g. change sides regularly, open windows to air the room regularly, open the cover if it comes with one, use a mattress protector to avoid liquids to pour in, etc.) it will not matter that much either; but if you live on a humid place well... then myself I wouldn't go for a foam-only mattress.

On the firmess you're right, it is very variable. I've been myself sleeping on a foam-only sofa-bed mattress for almost a year and it is firm enough. The issue with latex or foam is that they distort with time. It is unavoidable. Depending on the quality of what you have bought that might be years but I've seen many poorly made mattresses that are returned within months.

It also depends on the weight of the people that is going to sleep as well. My wife would be worried if you were a customer insisting on buying a foam mattress and your weight as a couple were bigger than 180kg for example. Specially if it is a wide mattress as the weight would not be distributed evenly.