Aspirin was found to have an effect on 15-year-mortality significant only at the .05 level, and aspirin was found not to have a significant effect 20-year-mortality, so take it with a grain of salt.

Can you provide your reference for this? I looked at the meta-analysis and what I assume is the 20-year follow-up of five RCTs (the citations seem to be paywalled), and both mention 20-year reduction in mortality without mentioning 15-year reductions or lack thereof.

Edit: Never mind, I found it, followed immediately by

the effect on post-trial deaths was diluted by a transient increase in risk of vascular death in the aspirin groups during the first year after completion of the trials (75 observed vs 46 expected, OR 1·69, 1·08–2·62, p=0·02), presumably due to withdrawal of trial aspirin.

I'd like to see 20-year numbers for people who maintained the trial (and am baffled that they didn't randomly select such a subgroup).