Probably in drinkers – because the increase in bacterial titer in the stomach and small intestine (due to the reduction in stomach acid, which with free fatty acids, is a potent germ killer, at least in vitro; we do not know if it works in vivo) means that more acetaldehyde will be produced from alcohol. Good luck proving it – this will only occur in drinkers, and acetaldehyde is but one additional carcinogen, and not the strongest one by far, and all drinkers make acetaldehyde from alcohol, especially in their mouths and colons – those on acid blockers probably make a little bit more in their stomachs and small intestines. A tiny, probably imperceptible increase in the odds of getting cancer. It is easier to find a needle in a haystack.
Nailing a drug requires a smoking gun – some big, hairy, obvious deleterious effect, not some nameless, faceless increase in the odds of any one or any number of bad things. That is why so many drugs continue to be used and abused despite the fact that chronic use of them is a bad idea.