If you think that simply having a bunch of heavily armored combat drones eliminates the need for actual combatants, then either you've never dealt with a halfway competent GM, or you're fragging crazy.
As with anything in Shadowrun, there are strengths and weaknesses to using drones. The biggest weakness of drones is that there is a whole archetype out there that focuses precisely on taking technological toys away from people and making them their own. They're called Hackers. Any group running around with only combat drones as their attack force is asking to have a hacker turn the drones on them and rip them to shreds. Or for a mage to drop a Powerbolt or six on them. Direct physical combat spells are MURDER on drones, with their relatively light Body scores.
The drones in 4th edition (especially those designed for combat) were hardy enough to not fall apart the first time someone looked at them wrong, but certainly never replaced the need for meat body combatants in any game I played in or witnessed. Indeed, it is my profound hope that they don't butcher the customization options for drones and vehicles like they did with weapons in 5th, but bring them forward mostly unchanged. For those that didn't mess with rigging in 4th, armor was limited by the Body of the vehicle/drone it was put on. Vehicles could have Body x2, while drones could get Body x3, all capped at a max of 20 for normal armor, and 10 for concealed or smart armor. And upgrading the armor took up mod slots, which were equal to the Body of the vehicle/drone. So a Steel Lynx (a mid-range combat drone) with its Body of 4 could max out armor at 12. So if you shot at that Lynx, it would get 16 dice to defend, for an average of ~5 hits. Considering that an Ares Alpha (a fairly common assault rifle) did 6P with normal ammo, then if you got net hits on the attack, you were pretty much sure of doing some damage to the drone. They weren't all powerful, but there were a legit threat, on both sides of the table. And tricking out drones with autosofts, weapons, and other modifications could run expensive enough that they most certainly weren't treated as cannon fodder unless it was a desperate situation.