I dont think that the 5.0 is going anywhere anytime soon. the mod motors will increase in popularity and aftermarket over time but i doubt that the HUGE aftermarket for the 5.0 is just gonna give up on a very popular motor.
I also believe that technology for the 5.0 may also increase. If you want an example of a high technology pushrod just look at the LS1 (i know its a chevy
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Im not too sure why a mod motor is more of a revver than a 5.0. I think they rev better stock because there are many people who rev 5.0's higher than mod motors.
I doubt we will ever see a pushrod 5.0 in a new production Mustang again but i dont think it is just gonna stop being around in fast cars. In my opinion the 5.0 will probably always be cheaper and easier to work on then a mod motor (but i would love to have a new GT
Later,
Nick