What guarantees that RTL inverter is in triode region when the input is high?
Try looking at a typical mosfet characteristic. here's a fairly generic one: -



Basically, dependent on the drain current, you need Vgs to be high enough to avoid the saturation region. So, when you think you might have a device that is OK read its data sheet and look for the above graph. They always have this graph in the data sheet.

Drain current is largely determined by the load resistor and the voltage on the gate. The voltage on the gate produces the shape of curve in the triode region which, for a given drain current is a reasonable approximation to a resistor - think about the top red curve and concentrate on the triode region - imagine it is a straight line i.e. a constant ratio between Id and Vds - this is often referred to as "the on-resistance" of a FET.