"it doesn't have the unquestionable dominance that it does in Search"
Did you read the links above? Google *is* dominating ads services. Facebook is second, at some distance. Just look at the revenues:
Do you believe they come just from the Search Page and YouTube?
"but only part of DoubleClick is relevant to display ads."
Of course, but that's what allows the other parts of the business run - like setting targets (thanks to Alphabets slurping operations), and analyzing ad campaign results. The advertising exchange of DoubleClick is surely a risk to display malware-ads.
Do you believe advertiser would buy the services if their ads are not displayed? Google has more to lose than others, if ad-blocking becomes widespread. Its whole business is built on them.