It would be highly unethical to promote non-registration IMO. The balance of power is not on the side of the individual.
It's already highly likely that many won't register: elderly in care homes or living with dementia, people with disabilities, people who don't have access to phones or digital media, people who don't realise that they need to apply - many older EU citizens who have been in the U.K. for decades seem to think their existing ILR is all they require.
In the home office trial, of the Roma that did the settled status process, only 2 individuals were able to manage the process without assistance (I'll try to find a link for that, but if I remember correctly that was only about 5% of the Roma involved in the trial).
I'd be surprised if the number of people who fail to apply is less than 10%, some won't do it for the above reasons, some because they actively refuse; some may prefer to leave rather than do it. And I'd also be surprised if the number of failed applications is much below 10%, because even if the HO assesses applications in good faith, as the process rolls out and the help available becomes over-stretched, there will be more incomplete applications or applications containing errors.
The most recent information I've seen suggests that nearly 10% of the applications from the first two pilot phases were still outstanding. And most of the pilot applications will have been straightforward cases.