This is a scandal - not just of the NHS, but also of the police who appeared to outsource their forensic work to a known grifter (no crime, no pay) with no forensic or neonatal experience and indeed the entire judicial system.
The notion that an expert's evidence cannot be reliable if they are paid is misleading. Of course they are paid - they can't live on air, and often the work involved is very complex and time-consuming.
Every professional witness giving evidence is paid. That includes, for example people giving evidence in personal injury cases, disputes about alleged negligence in building or in industry, tribunal disputes etc. It doesn't mean that it has to be assumed that they are biased in favour of the person paying them. Expert witnesses are generally subject to their own professional obligations and ethics, and in the case of medical witnesses that means a very strong obligation to tell the whole truth, not to suppress any information which might seem inconvenient, and to give an honest, fully reasoned opinion based on the available evidence. It is also not in their interests to depart from those standards, because not only will there be witnesses called by the other side who will almost inevitably pounce on any weaknesses or untruths, there are lawyers and judges who will go through their reports in great detail and are very experienced in testing these things. If they regularly give biased evidence that fact would become obvious very quickly, and they would basically never be used again because there is no point calling an expert witness who is well known to be totally unreliable.
It really is very telling that no expert witnesses were called on Letby's side. I'm quite sure that her lawyers will have tried to find people to discredit the prosecution evidence, including the statistical evidence, but clearly they couldn't find them - or the people they did find actually backed up the prosecution. That's another illustration of the fact that experts don't just say what their paymasters want to hear.
I don't know anything about Evans, but just calling him a grifter because he was paid for his work and therefore he is unreliable just isn't rational. It must have been open to Letby's lawyers to give all the reasons why his evidence might not be reliable, but you are left with the fact that, despite strong cross examination by a very able KC, the jury still apparently accepted his evidence.