In science guess who has the burden?
In a legal context the burden of proof is broken down into the burden of production and the burden of persuasion. If you fail to meet the burden of production it's fair to say a claim is bullsh*t. It necessarily follows that if you haven't met the burden of production you cannot have met the burden of persuasion as a logical matter.
Remember, any claim must stand on its own terms. It doesn't matter what is the reality or truth of the matter. We can never know the absolute truth of something. We can only attain a slightly firmer grip on reality when people propose persuasive arguments. Maybe you can mine a failed argument for useful bits, but a good rule of thumb is that it's not worth the time if the argument cannot even meet a minimal burden. If it wasn't worth the claimant's time why would it be worth yours? That's strong circumstantial evidence to move on.
Lawyers can drop clients with cases that they think are losers and refuse to take them to trial. And, very, very often, before or directly after closing arguments, if the writing is on the wall, a case settles.
Additionally, what sorts of cases are we talking about? Insurance defense? Toxic torts? Data breach?
The 'winnability' of these cases varies highly. There are many, many law firms who's entire business model is built entirely only on taking no-brainer winning cases. Think injury lawyers. They may - perversely - have very low winning percentages, because 90-95% of their cases settle, and only the real squeakers get to trial.
Let's look at the inverse. Do you really need Quinn Emmanuel or Gibson Dunn if you have a slam-dunk case? Or do you need the best litigators around when your case is a total coin-toss? And, in that event, is it an example of bad lawyering if your crack-team loses because they are pushing the bounds of appellate advocacy?
The idea of judging 'law firms' without further taxonomic distinction, generally, by trial disposition is just - it is frankly idiotic.
Furthermore, on average the effects you are mentioning will wash-out, unless there is a systematic bias whereby lower ranking firms and higher ranking firms settle in different manners.
All real-world analysis (especially for observational studies) rests on certain assumptions. It is always true that these assumptions might be wrong, but it is important to think about whether or not the assumption is plausible. It seems plausible that on average, a lawyer who is able to get better outcomes when they don't settle is also able to get better outcomes when they do settle.
Furthermore, even if most cases are settled, the rare cases that do go to trial can have an outsized impact. Usually people settle because a bad judgment is devastating (as well as not wanting to pay legal costs).