Actually, a depressed trajectory is selected to give maximum range to the missile. According to extremely basic principles of projectile motion, the angle of launch should be 45° for the range to be maximum.
Pakistan does not have any known missiles of ranges longer than 2750 km (i.e. the range of th Shaheen-III). But this is because we don't need them. However, our missile program is a dynamic and ongoing program. Not only are our weapon designers constantly improving the quality of our systems, they are also gaining valuable experience and technical data which would make the design and development of longer range and more advanced missiles easier.
Also, I wouldn't be surprised if Pakistan already has developed an ICBM capability but is waiting for the right geopolitical situation in order to test it.
Remember, the indication that the Shaheen-III existed was first given by then-Chairman NESCOM, Dr. Samar Mubarakmand in 2004. And the speculation about MIRV capability had been around since at least 2003/2004, when the Shaheen-II was first tested. In that vein, Pakistan may have already completed the design studies and prototyping for a future ICBM and development may be going on right now if it has not already been completed.