Firstly, I would be the last person to say that everything in the military is perfect, in fact, in my critique of my institution lies my great respect for their decision to distance themselves from public affairs and politics. Its no mean feat that's been achieved after decades of struggle by progressive minded officers who want to see Pakistan as a strong, independent and viable nation state that is not dependent on any particular group to serve as crutches.
However I disagree with your concern on promotions, yes there is some politics involved but that is way up in the higher echelon when you're playing with the stars. Anything below it is strictly merit based, there's noway anyone can be "passed up for promotion by some privileged son of a major", there are not a set number of promotions to be handed out that must be allocated accordingly, instead its an open field with the board promoting as many people as they see fit for continued service.
Lastly, I can understand civilians wanting to see a coup, but then they want to see a civilian setup just as fast. A system of governance cannot simply be installed like a computer program, it needs decades to evolve and mature. This understanding has finally taken root in the Army and it will help strengthen this country immensely.
P.S: I am the son of a retired General stuck in a no upward mobility, mid-tier posting after being injured during training. Agar meray abba meray liye pawa nahi laga saktay toh Major ki khett ki mooli hai, jisko abhi tak strategy ka lafz istemal karnay ka ikhtyar nahi mila fauj main