Very pretty but also a very non accurate picture has been painted here. I use 30mm un supported rails on my machine and the accuracy is as good as can be (way more than my needs). The main factor is that it is all bolted to a very sturdy structure. the problem with your assumption here is that the rails sit on something with a solid base. The main problem with machines is the deflection of the bed and not necessarily the rails? if you look at all those nice machines with expensive supported rails or slides, most of them are mounted on aluminium profile that will flex more than the rail. You need to recalculate this to show the defection of the bed for it to stand up? Also what is this for Milling or routing?

Thats my 2 pence worth