James Anderson has retired. Jofra Archer is a shadow of his former self. Mark Wood hasn't played any form of cricket for the last 6 months or so. Chris Woakes was also missing from the squad. Given all this, I would say that the poor performance from the England bowlers was more or less expected. Brydon Carse and Saqib Mahmood did their best, but they are not of the same league as Wood or Woakes. After the South Africa series, England will play a bilateral against Ireland. The younger bowlers may feel more comfortable in that series.
It is true that team is missing key players so their poor performance is not surprising. And that is right that James Anderson retirement damage performance of team little. Your observation abou absence of Jofra Archer and Mark Wood and Chris Woakes is correct as is your point that younger bowlers like Brydon Carse and Saqib Mahmood are promising but still developing. As you said upcoming series against Ireland will be good chance for them to gain experience.