Antonym for "evangelist" (in modern/technology industry usage)

Your word, evangelist is synonymous or at least 'homoionymous' with missionary and champion. The words you have chosen could do. The words you have chosen might do, though have an artificial and slightly heavy ring to them.

In the context that you mention, surely the polar opposite would be 'sceptic' or 'critic'. The evangelist is literally a messenger (Greek αγγελος - angelos) spreading good (eu) news. The opposite is one who casts doubt on the news in question.

There is another (biblical) word, generally used critically, that applies to someone given to saying that things will turn out badly. Such people are often called 'Jeremiahs', after the prophet of doom in the Book of Jeremiah. But I would suggest 'critic' or 'sceptic.

Naysayer may work. From Merriam-Webster:

: one who denies, refuses, opposes, or is skeptical or cynical about something

(example) There are always naysayers who say it can't be done.

So if an evangelist promotes a product, a service, or an organization, a naysayer vocally opposes the product, position, or organization. For instance, a 2010 PC World article reported "Windows Phone 7 Buzz Muted by Naysayers," with this body text:

In fact Microsoft is facing a wall of naysayers leading up this Monday when it will introduce its first batch of Windows Phone 7 phones. Analysts and investors are wary about Windows Phone 7's prospects. Third-party application developers are giving mixed reviews about the new smartphone platform. And average Joes and Janes are just making wisecracks.