Tbh that's how you build a strong bench in any format, new guys need more experience in international too and this series wasn't high profile so it make sense.
First choice players need some breaks as well, as most of them just took part in PSL so the board needs to take care of the player's workload.
Afghanistan over the time has emerged as one of arch rivals of Pakistan in cricket along with India. You can see the intensity of heat whenever Afghanistan play with Pakistan. That's why people is Pakistan are upset because of this decision that resulted in loss of series.
You know the biggest beneficiary of this series loss? Muhammad Rizwan. He was criticized for his slow Strike rate in T20I and now he is all set to make a come back for at least a year. Pakistan team is full of giving surprises and this is just another surprise.
Before this series loss, it was a genuinely one-sided rivalry from Afghanistan's side, similar to the Ind-Pak rivalry. To my knowledge, in the last decade or so Indian team (at least fans) never took the Pakistan team very seriously in WC or other ICC tournaments because the winning gap was too big (the score was 0 to 13/14 in the ICC tourney) but this mindset changed in the Champions trophy finals (2017) a bit. So I would advise that don't dwell on this series too much, move on and take this as an experiment.
It's up to selectors if they go with the Rizwan but we don't have T-20 WC this year so they have a window to improve their gameplay. Having said that IMHO Rizwan and Babar are the worst T-20 batters in the modern world so I won't defend them.