I disagree slightly. While, in general, you're correct - the culture war can only be won by building and sustaining cultural norms as well as enshrining, in law, policies that would bolster that culture - I believe that memes act as a brainstorming session for everyone to present their points of view and hone their argumentation abilities. In doing so, you never know who might read your meme with its follow on comment section and see something that speaks to them, challenges them or changes their mind entirely.
The major benefit is that memes, when you're doing it right, are supposed to be funny. Therefore, one can get their point across without the message coming across as preachy. A substantial number of the /politics memes fail in this regard, sadly... and I include myself in that comment. But memes and light-hearted discourse absolutely have an impact - to a greater or lesser degree - on the cultural landscape.
Does that make any sense or am I just seeing hope where there is none?