I am speaking from a position of complete ignorance here, but I would be very surprised if the number of people refused an ESA/ETIAS doesn't end up being a lot larger than the numbers being refused entry under the current system.
It seems a bit misleading to say that they transfer questions to pre departure when often these days there are no questions. And I suspect that there will be questions (e.g. on criminal records) that I don't think are usually asked at present (certainly not in my - limited - experience anywyay).
It should be a fairly similar refusal rate when you factor in people who are issued a visa after being a refused an ETA. These are essentially the people who get interrogated at the border, are asked to show proof. High income countries tend to share serious criminal convictions with each other (or just publish them). If John Jones turns up at St Pancras, when his passport is scanned then the French border force system should identify his 3 year prison sentence in UK for drug dealing. When ETIAS launches, if he is stupid enough to lie then they can just ban him for deception and not bother considering whether to let him in. I have been asked for criminal history (none!) on landing cards entering multiple non EU countries. The big risk is that the list of questions could keep getting longer and then the red line is crossed of asking for supporting evidence, that's when it becomes an E-Visa.
I seriously doubt the security model of the UK ETA, especially as when I applied for mine the only question I had to answer was my job title. I don't know whether those questions are changed based on nationality or other factors.
The UK ETA is more or less just trying to identify whether you are someone who is already of interest. You will have also provided your name, date of birth, place of birth, current address and some contact details. Thats enough to identify you if you are on a database for negative reasons
As
@AdamWW highlights there is a little bit more to it but that is the gist.
The way I view ETA systems is that they are the fourth stage of evolution in border controls:
1st - Pre IT
2nd stage - big IT systems with a border officer manual inputting your details and doing some searches (1980s to early 2000s)
3rd - Use of biometric chips to hold passport data and load information without border officer having to input it. Early 00s onwards.
4th - using NFC chips in phones and apps to gather the data to enable checks to be conducted pre departure. Late 10s onwards (Europe is behind).
The latest tech starts to open up being able to remove gates entirely but it is reliant on the person providing the information pre departure.