And foreign intelligence agencies. The agency will win, and you will lose. If the agency has a mandate, then it will accomplish that mandate. No matter what. Period. End of game. It's just the methods that differ from nation state to nation state.
Generally, I agree with this assessment and I am a pretty good example of it in action. If I cannot fully secure a network from all known and unknown threats then it is incredibly unlikely anyone else can. I'm not bragging, but it's my specialty, it's my engineering degree, it's my engineering certifications and it's 2 decades of training in it. If they do want in, they'll eventually get in. If not initially, then they will patiently await your next mistake and be ready to exploit it.
The first and primary method is to probe your network, gather information and utilize that intelligence to formulate an attack plan. They'll find a vector of attack, if not immediately, they'll find one shortly or an up and coming one you neglected to consider. Most of the off-the-shelf stuff amounts to playing security theater in the end IMO. Generally, you are dealing with entities that have virtually unlimited resources. They can throw millions at a company that owns the software you use if they need to. Your security theater amounts to speed bumps for the eventuality you'll be compromised, slow them down, make things difficult, sure and good job but - all speed bumps in the end.
They'll go after your weakest commodity. The kids, the wife, the uncle that visits every couple of weeks. Your weakest link will be the vector of attack. They won't start with you because you are smart, you are crafty, you know counter intelligence and know how to secure your personal machine. It's Uncle Bill or your child that is the doorway into further information gathering and surveillance. You likely don't have the time or energy to secure everyone around you, nor do you want to deal with the headaches involved or the inconveniences to those people with a severely locked down lifestyle.
Now the question is - will they expend the effort? Probably not in the majority of cases. But if you detect them, then stop it, they'll wonder why you are worried about it or what you are hiding and it will escalate the matter. Once escalated, if you continue to present hardships and roadblocks, it will escalate it further along and suddenly assets and money will be pushed into your direction. A cold war develops and continues to escalate. At which point you need to decide, continue to play the game, give up, or perhaps use your own counter intelligence and start feeding them fake information or make it appear like you made mistakes and they are seeing 'everything' when they really aren't? In all of those cases they'll eventually unlatch from you and revert back to passive surveillance.
It is fully possible to secure your personal information from virtually all forms of spying/intrusion. We know this. It is virtually impossible to fully secure your network from all possible intrusion methods. We also know this. So you must find balance.. Secure your most sensitive information in ways you can virtually guarantee it will be safe, that'll keep kiddies, under funded actors, well funded actors and targeted surveillance out of your loot.. Secure your network using accepted practices, that'll keep the kiddies and under funded state actors out, but accept the inevitable that no network can be totally secured from all vectors from well funded state actors.
Carry on.