Re: too late
Yes, it is too late in at least one field.
Anti-missile systems are already exactly this, because they don't work if they aren't - see HMS Sheffield for a tragic example of why humans can't do anti-missile.
You select a volume of space for them to check, the locations to protect and turn them on.
They then automatically fire upon and (hopefully) destroy any incoming items they recognise as an inbound missile.
There is no human in the loop, because by the time the meat-bag hears the alarm, it's too late for an anti-missile system to do anything.
Now, it's probable that the operator can order the anti-missile-missile to self-destruct after it's launched, but it's still very little time.
On the other hand, there is a difference between anti-missile systems and anti-tank etc. as there is a lot more time to identify the target before you need to open fire - although still not very much.
On the third hand, what are the military supposed to be defending against anyway?
Who has tanks and might invade a neighbour in the next fifty years? North Korea and Iran are about it!
The current and near-future threats are individuals or small to medium-sized groups (perhaps associated with international movements), not states.
Autonomous systems can't identify those, and really, neither can military personnel either - although they usually do better.
It's effective policing and peacekeeping forces that are really needed these days.