"We announced during the Chrome Dev Summit in 2015 that when Autofill is enabled we see 25% more forms submitted than when it's disabled [3]."
In other words, 25% more users will make it to the next stage of your conversion funnel if autofill is permitted.
That's a reason to turn on autocomplete in your signup form, not to force it on in the browser.
That would be a negative user experience, given the prevalence of legacy forms on the web. Example: "Why does this autocomplete feature fail most of the time?"
"and of you're using a form for something other than a conversion funnel, well screw you"