The phrase 'merely so' is correct and can be used in written English.
It is often used to emphasize a point or to join two different ideas together to give a sense of completion. For example: I couldn't believe he got away with it - merely so with a simple apology.
As with Wodehouse, any semblance of a plot exists merely so that the characters have something to do with their hands while they spout funny things.
But Mike Jr. soon realized that he did not want to work full days as North Penn's audio-visual coordinator merely so he could coach football after school.
The double domestic situation became known to Tomlinson only when he was an established actor and merely, so he said, irritated by his father's pretences.
TRACEY ULLMAN did not become an American citizen merely so she could make fun of the residents of these United States.
One of those who attended wrote an abject letter explaining that he had gone merely "so as not to ostracise Miss Jakobsen".
Most young men seem to grow beards merely so they don't have to shave, so you get these ghastly Grimms' fairytales things that have most of yesterday's Pret a Manger in them.
Some of my favorite of his jokes are merely so-so, because, almost as if he knows it, he really sells the presentation.
The rest is merely so-so.-THORNTON WILDER.
Casey Broadwater of Blu-Ray.com stated that the episode "is merely so-so entry in the show's annual Halloween anthology", while Colin Jacobsson of DVD Movie Guide wrote that, while it "doesn't present many significant flaws", it also "doesn't ever really shine".
The timetable was "not merely challenging but so tight that it may risk endangering the quality", it said.
It is not merely that so many New Yorkers have a lot, but rather that so many have heard about way too much.
