This is one of the common fallacies which are repeated about this case; it was not an uptick, increase, blip, spike, etc, that caused the deaths to be investigated initially. It was the fact that these things were happening completely out of the blue and doctors (and other staff) were noticing that Lucy Letby was always close by and usually directly involved. It was being noticed as it happened and not after the fact, weeks or months down the line when the stats were crunched.
Also, although these babies were indeed very small, they were not all "sick" as such. They were premature but were not sickly other than that and were not expected to suddenly collapse or die.
The unit was not optimally staffed but that's a common story in the NHS - and probably common in every healthcare setting other than expensive private hospitals. In any event, you still need to associate the actual events with a causative link as to how, precisely, that understaffing lead to the deaths. No one has done that so it's nothing more than coincidence and coincidence is not causation.
Once again to repeat; most of the folks on here have been looking at this from the very start and have followed every single day of the trial. A few even attended it. Unless you have a very detailed knowledge of the whole trial then I would really suggest that you start reading this discussion from the start rather than reading the cherry-picked media reports. The Daily Mail podcasts are very good as well and are definitely worth a listen to.