First of all, David-kun is a gag manga and afaik sales don't matter as much for a gag manga. Especially considering it's constantly switching last place with Jimoto and last place is usually reserved for gag manga. Idk the exact sales of other series that used to be a gag manga in last place, but I doubt they were that high.
Btw, Jimoto is selling even far worse than David-kun. While it is getting an anime, that's not because of popularity. We can assume both are competing with each other and I don't think just because Jimoto is getting an anime, it necessarily has the edge over David-kun. Because the anime is the only thing that speaks for Jimoto. It can just as well be moved to SJ+ or the manga can get cancelled and it continues as an original anime. I mean there isn't much for the anime to adapt anyways, so most of it will probably be original anyways.
The way I see it, David has the edge over Jimoto despite Jimoto receiving an anime. Also, Jimoto's concept from what I've heard is very limited because every chapter seems to be based on a prefecture in Japan. So once there aren't any prefectures left, the manga might be over. Iirc the mangaka even said he doesn't know what to do after he covered all prefectures.