^^It doesn't quite work like that. Really, guns today are deadly enough, but that still doesn't mean anyone can pick one up and be very effective with it. A dedicated ground force division of the military will still be a necessity regardless how the advancements made in weapons deployment technology.
Guns are not phasers, just like firearms are not slings, bows and crossbows...

Problem with guns is that you have to teach people how to use them; you have to know how to aim accurately and how to deal with recoil, how to fast reload etc. All of those problems doesn't exist when we are talking about phasers.
Wide beam setting, made aiming far less important. There are even some people who belive that phasers are capable of auto-aiming, but whatever or not, this is true, is open to debate. There is no recoil, and you don't need to carry ammunition whit you.Sure, soldiers know how to deal with firearms because they were extensively trained to use them, but only minimal training is necessary to use phasers. So, again why do you need professional soldiers?