I'm pretty sure I know people who have been convinced by meme quotes. A headshot of a politician they don't like, with text overlaid, which they never said. People are outraged! And never bother to inspect the source.
Anything that makes it easier to lie about what someone said or did, or makes it harder to disprove... They're all politically weaponized, already.
Look at the "drunk pelosi" video.
Technology is evolving but not in a vacuum, society's reactions also evolve in response. Today, many people interpret video to be "evidence" but those same people can interpret a photo to be a "claim" or perhaps some form of lower-confidence indication. Before photo manipulation was commonly known, I think photos were in a similar place as video - more trusted. Based on history, it's reasonable to expect video may follow a similar trajectory as photos to becoming less trusted in situations where it matters.
So, what happens when media types which were previously more trusted as evidence become less trusted? The same things that happened with print, audio and photos. Viewers will evaluate external cues such as the reputation of the publisher and corroborating evidence. The leading indicators that we should suspect deception will likely be similar. For example, how divergent the behavior depicted is from expectation, how contentious the surrounding context is and the existence of parties with an interest in creating such a deception.
This effect already happens with manipulation of intent through tricky video editing, for example deleting the rest of a reply to a question or even swapping in an alternate question. In the last decade I'd say the typical person is far more aware this is possible.
So, in the near-term there may be some successful deception but in the long-term I expect the potential value of creating such deceptions will diminish and we'll arrive at a new "normal" much like we have now. The biggest long-term impact may be false claims of "doctored video!" from those who were actually caught on video doing something they didn't want seen by others. But as we already see now, those pre-disposed to believe whatever is shown is false will search for indications it's doctored. Those pre-disposed to believe whatever is shown is true will search for indications it's just more confirmation of what what they already suspected. Either way, the existing reputation of the person shown, the distribution source and the pre-existing knowledge of viewers will likely be more determinitive than the media itself.
It's hard to predict actual effects, I don't think that anyone could have foreseen that the primary use of stills image editing for manipulation is not the perfect crime of an elaborate fake but a barrage of provocatively simple memes that don't even pretend to care about believability. The act of sharing is the message.
30 years ago, that might have been the case with doctoring images, now practically everyone has a personal computer that they can install and use PhotoShop or some similar tool on.
I really wonder which type of meme has the most influence on average, straightforward and outright lies like the one you've noted, or the more subtle, subversive social commentary style. I'm a big fan of the latter, I think they're very interesting and underappreciated.
For example, this one - nothing more than a simple screenshot of Twitter, but to me this seems very persuasive: http://magaimg.net/img/80rb.jpg
Pointing out hypocritical grandstanding: https://i.redd.it/74n4exoy2g131.jpg
Media bias: https://i.redd.it/gtrq4xgemtx21.jpg
If Biden runs will be interesting to see how much airplay this meme will get: https://i.imgur.com/tqzGS6E.mp4
Laughing at the silliness of popular narratives: https://i.redd.it/a9wrex3ijc231.jpg
Just for a laugh: https://i.redd.it/i5bvml5m92131.jpg
Laughing at logical inconsistency: https://i.redd.it/rp9ydf0s88y21.jpg
An interesting way of looking at Brexit: https://i.redd.it/cs6o72p3dp031.png
Historical hypocrisy on border control: https://i.redd.it/983sm9wjley21.png
All of these are from t_d so obviously one-sided, I'm sure a similarly impressive collection from the other perspective could easily be assembled, and it's not that uncommon to encounter otherwise intelligent people who have obviously had their beliefs shaped by those memes.
I'm now just about ready to unplug the whole thing and launch it at the sun.
To your point, there are people who are persuaded by assertions.
I personally find it gets even more entrenched when people believe they have seen the evidence with their own eyes. When they see a doctored photo, clip out of context, etc.
I'm reminded of the gun / water context photo:
https://toinformistoinfluence.files.wordpress.com/2012/06/19...
Oh ya I love that picture, was trying to find it not that long ago with no luck. It does a brilliant job communicating how powerful propaganda can be.