Hi again Erin
I agree with your conceptual diagram. However, if the risk from young age, non-diabetic status, non-smoking status and normotension was 4% and that from pre-treatment LDL-C was 5% then on the additive scale the total risk would be 4%+5% = 9%. On a statin the new risk would be still 4% from the 4 risk factors but 5% x 0.6 = 3% from a risk reduction of 0.6 from high dose statin. Therefore, the new risk by applying a risk reduction on the additive scale would be 4%+3% = 7%. However, by applying the risk reduction to all risk factors, the new risk on statin treatment would be 0.6(4%+5%) = 4% x0.6 + 5%x0.6 = 2.4%+3% = 5.4%, lower than the 7% and therefore exaggerating the risk reduction in that patient.
I am not saying that statins are not important in preventing vascular events or do not work. Far from it. If I was presented with someone with a high risk of vascular event and a high HbA1c, a high BP but a low LDL-cholesterol, then I would persuade the patient to allow me to help to improve diabetic and BP control and focus on this at least initially. I do not believe that much would be gained from offering a statin as the risk reduction would be small when calculated on the additive scale (unless there is a RCT that contradicts this). I would not burden the patient with another pill. However, if the LDL-cholesterol was also high or the patient was already known to have had a vascular event, then I would add a statin because the expected risk reduction would be greater. Also, if a 75-year-old person with an inevitably high risk of a vascular event in the next 10 years had a low LDL-cholesterol, low BP, and no diabetes etc, I would not recommend medication.
As far as I understand, this is how most physicians would have been reasoning over the past 25 years. In this sense they would not assume that they should treat a high risk of vascular event with a statin irrespective of its cause(s). You seem to assume that there is a very strong interaction between non-lipid based risk factors (e.g. age) and lipid-based risk factors, which should be modelled by an effect on all the risk factors (which if untrue would potentially increase sales of statins unjustifiably). However, as far as I can understand, the way that multivariable risk factors are calculated assumes that there is no such interaction at all so that there is independence between risk factors (maybe @f2harrell could comment on this). Perhaps the truth lies in between. Establishing such a truth would be very difficult as we discovered by looking at the literature previously (Risk based treatment and the validity of scales of effect - #12 by HuwLlewelyn).