Until the partner gets out, after which they're no longer required to just survive but to also score runs. Which of Abbas, Shaheen and Naseem look capable of even hitting double figures in 30% of their innings?
There is a way to develop tail. They need basically two things - solid defence when playing with a top order and using long handle almost hopelessly when alone. And yes, often they’ll fail in first task, most times in second one.
Now, you can’t train them to a good batsman,‘can’t over burden them to become posedu all rounder and sacrifice bowling, can’t give longer time in National team’s net session because they need to bowl to batsmen. They just are not skilled enough to do the job - it’ll be foolish to think that Machrath or Murali didn’t have the so called “talent”, but they just wasn’t equipped to use that for batting. Same can be said for batsmen as well.
But, what you can do is to develop them marginally better batsman than what they are by teaching few tricks and inserting some common sense. I mean, you don’t need to a genius not to play that shot what Yasir played when Rizwan was well set at other end. The best way to do that, ad the Aussies will tell you - use the domestic FC cricket. Make their respective FC teams to put Shaheen, Naseem (& few others) at 8 or even at 7; may be Yasir & Faheem at 7 or even 6; Dhafab at 5 or 4.... regardless of the batting potentials of other bowlers and force them to bat n the middle for longer. Second thing is, associate a mentor with the bowler from his team mates - Waugh Sr was responsible for McGrath, Waugh Jr. for Warne .... these were their FC mate as well - almost for whole year a running mentor who’ll help them to be more effective tailender.
Of the four, Yasir had the best potential and he is a Test specialist, played 5-7 games per year, should have worked his way into a 20 average batsman and he had the talent, but not hunger. Of the three you mentioned,’best organised batsman is Abbas, but he has max 2-3 years shelf life. Naseem is the poster boy and a perfect reflection of modern PAK team - all shine but not that sharp. He MUST improve his batting to bat at nine because like Yasir, he should be used as Test specialist only. Shaheen is more organised than Naseem & can use the long handle, but he is already over burdened with cricket in three formats, at two meterish height, it’s not safe either for him to train too much for batting - he should be allowed to gradually go down as no 11. Still can use long handle and whatever comes is bonus, he should focus to become a pacer somewhere near Michel Starc with ball and Mcgrath with bat - that’ll be like plutonium for PCT.
