Actually, you can. It’s an agreement between two players and working out tactics against much smarter opponent plays a huge role in learning the game. Don’t tell how brother should play chess with younger brother. 
I’ve been the younger brother years ago and I still play chess. Most probably I wouldn’t, if he beat me every match, as that’s the case of table football I still hate. Here, real life example and perfect analogy. It’s still cheating, but serves the learning process well.
Back on topic, I don’t buy it. Comparison to previous titles isn’t fair as you ignore all the successful mods that made these, and other, games constatly difficult (yet fair) till very end.
How I see it, is that in PP invasion progression is very, very basic. In previous titles, as the game progress, it throws bigger and badder things at you. Small scouts turns medium and large, terror and battleships are more often, their crew is upgraded as well. In PP DDA tries to fill the role of progression and just tries to make you feel like enemy is doing something to stop you because there’s nothing else enemy can do. Enemies are mostly the same since early-mid game, there is hardly anything new as the game progress. No real enemy progression means no need for technological advancement (and hence the lack of such in the game) over the course of campaign to overcome new obstacles.
It would be great to have an option to turn off DDA but I’m afraid it’s not going to happen, as there’s just no content to fill the gap.
This, being the core of game progression is so… disappointing.