I worked with one of the BIG defense contractors for a while. There was this guy, Willy. Willy was a 'fellow' with the company, an engineer so advanced and critical to the company that they just let him do whatever he wanted. One of his PhDs was in chemistry, I think he was about 80 years old back then (~2010). He did not use email at all. I thought it was crazy at the time, but man, was WIlly right about that one. If you wanted to see him, you'd have to go over to him and talk or drop off the memo. He had a phone, but it was the bar down the street from the campus, as Willy was usually there, watching a game or working on something.
I remember WIlly telling us about a time, back in the Cold War, when he'd be given briefcases with empty aluminum bottles inside. They'd tell him to measure the air inside and tell them if he found anything strange about that air, chemically speaking. He didn't say if he had discovered anything strange, though.
WIlly's general expertise was ablative nose-cone chemistry, for re-entry. In fact, as he told it, his brain was the entire field of ablative nose-cone chemistry; at least in The West. He never wrote anything down about it, at least not much. It was all in his head.
This is what Willy had on the company and he had them over a barrel. I came to find out that 'fellows' were just guys that had retired, had never written anything down, and then HAD to be re-hired, at any cost. They were the only non-soviets that knew the engineering about so much of what the company did. That's why Willy could just not do email and have a bar as his phone number and be a general asshat about things. He was literally untouchable.
In fact, there were a lot of 'fellows' at the company. And it bled them dry [0].
In my view, that is why so much of the US's defense tech is going 'backwards'. There just isn't the work force. They called it the 'silver tsunami' [1]. Basically, the US hired for the Cold War and just really never hired again. That's why drones have been so big in the last few years: no humans on board means easier reqs to go through so that even undergrads can get the work done. The same is somewhat true in space and all the other aspects of warfighting.
So it's not just that the low-hanging fruit is already plucked, though I totally agree with you. It's that there just isn't the workforce anymore. It's a complicated issue, so it's going to have a lot of things that are 'wrong' with it and no one aspect is THE problem.
That said, the cyberwar has been going on for a while now. I'd look at CRISPR-CAS9 and optogenetics as the bleeding edge frontiers to get into.
[0] https://www.bizjournals.com/philadelphia/blog/real-estate/20...
[1] https://en.wikipedia.org/wiki/The_Silver_Tsunami