Chinese is a language without case markings on words, but it does make some steps in that direction. It sometimes marks what in some languages would be an accusative or oblique case noun with 把/将 and leaves it in front of the verb. What is the motivation for this? Speech rhythm? Effect? Emphasis? I believed all such sentences could be rewritten without 把, but maybe this one can't?
As far as I know, 当成 means 'regard as, treat as, consider as' if a person is mentioned, or 'regarded as, treated as, considered as' if a person is not mentioned, which implies a third party regarding 1 object as something else, here '戏剧‘ as ‘时髦’
Therefore, '，就是把戏剧当成一种时髦。' is a passive construction, neither '戏剧‘ nor '时髦‘ are regarding anything, a person is doing that.
Theatre was regarded as a vogue.
Why would you say 1.'，就是把戏剧当成一种时髦。' rather than 2.‘，就是戏剧当成一种时髦。‘ We know 'theatre' cannot regard things, people do that.
Is 2. wrong? Can I rewrite 3. 你把我当成什么人? as '你当成我（为）什么人？ In 3. we see the 'regarder', 你， whereas in 1. the 'regarder' is not mentioned.
It is said that 把 marks the 'object' of a verb, in which case ，'把戏剧当成一种时髦' has 2 objects and no subject, so to speak, namely '戏剧‘ and '时髦‘。 The person or persons 'regarding' it are not mentioned.
However, even for me as a 老外， ‘就是当成戏剧一种时髦’ sounds terrible. Maybe '就是人们当成戏剧为一种时髦‘ is acceptable.
Rule: When the regarder is not mentioned, fill the gap before the verb with 把X， where X is the thing regarded.
Does that account for this syntax??