But you can't accurately judge the risks. How do you know that the risks and rewards of monamory aren't generally worse/less than the risks and rewards of polyamory?

Rarely in life can we accurately and precisely measure the risks and rewards. ;-)

I estimate the risks and rewards as best I can... incorporating the risk that I may be wrong about the precise value of the risks and rewards. *grin*

but if we just treat the relationships as separate objects that don't directly affect each other (or all people don't interact with each other - which is a perfectly reasonable scenario) then it seems to me that in fact the chances of a person being in a good relationship at any given time are increased!

Ah, but there is also a much greater chance that at least one of the relationships will be going up in flames.