Thanks for your answers Fab, and this link of discussion.
Sure, a kind of turret incorporated to the HQ, with a possible upgrade for stealth detection, could do the trick to avoid the player just selling it and build a bigger rush force at the beginning. That's a bigger amount of work to achieve, we understand that.
Still, imho a defensive capacity could stay the result of a strategic choice the player has to pay for. After all, all of us are not specially rush fans, but the game has to be fair for everyone and allow all strategies, even the early all-ins. In that case, this "HQ turret" could by itself, be an upgrade, available at start in the HQ. We would have to pay something, like 400-500$ for it by clicking on an HQ icon, and then the turret would quickly pop out on the HQ's roof. Why not with a further detection upgrade or else.
Yet we could wonder if it wouldn't take the place of the already existing turrets, even if those last ones can be built further to protect another zone...or if it wouldn't lead the player to build a "pack" of buildings right next to the HQ, later being an easy target for mortars, arty or air units...you'll figure what's best.
Then, what about the HP levels of units, or their price curves? higher prices but higher HP levels for all units couldn't smooth the traits of those easy early rushes? less units would rush at start, but higher HP levels could buy the players some more time to fight, either for the attacking or defensive player. So, as an example among others, infantry units wouldn't die immediately in front of a 2 Humvee rush, just placed in front of the Barracks exit...Soldiers would have the time to return fire once or twice before dying, while the following unit is built and goes out of the barracks. It could advantage the depth of the combat part somehow.
Your call for sure.
That's the saddest part of the story. I was getting used to think, after nearly three decades of its existence, that the word "Strategy" was the main cornerstone of the RTS genre and golden age.