Opposite of "trendy" without a negative meaning

If you're not looking for outmoded or unfashionable, I suggest "commonplace".

• Service Oriented Architecture is the trend as of now. But what is trendy in 2014 can become commonplace in 2015"

commonplace (adj) - an idea, expression, remark, etc., that is not new. Merriam-Webster

I think standard may fit the context suggested:

• commonly used or supplied; "standard car equipment", "a standard service".

If without a negative connotation I think the word classic is appropriate.

Normal.

"(In computing) Service Oriented Architecture is the trend as of now. But what is trendy in 2014 can become normal in 2015"

If I could impose some stylistic changes, I might phrase the sentence as follows:

"(In computing) Service Oriented Architecture is the trend as of now. But what is trendy in 2014 may merely be normal by 2015"

The change is to emphasize the change in status from flashy to the status quo. Without knowing more about what you intend to convey, however, I can't say if this is the right choice for you.