4. Most Arrowverse shows ran too long
While the Arrowverse shows did have too many episodes in their earlier seasons, which made telling coherent stories a bit of struggle two or three seasons in, we're not here to talk about that right now. For the Arrowverse's biggest issues with quantity over quality often came from the fact that the vast majority of these shows just ran too long.
The Flash ran for nine seasons and that's an amazing accomplishment, but if you watch all nine of them, you'll notice a considerable drop in quality at around its fourth season. By its ninth season, it was completely unrecognizable from the show that it once was. On the other hand, while Arrow managed to bow out strong with an incredible final season, some of the ones in the midst of its run weren't very good.
This isn't just an Arrowverse problem, as it's been an ongoing issue with broadcast television for years. If something is successful, it will continue to go on and on - even to its own detriment. And as the nature of the DC TV shows allow for a procedural element to them (villain-of-the-week), that's exactly what happened. In doing so, most of them lost their way multiple times throughout their long runs.
It wasn't an issue with every one of the shows, as Batwoman and Black Lightning never lasted beyond a few seasons, and DC's Legends of Tomorrow enjoyed a mostly consistent run, but the rest of the Arrowverse shows ran out of steam at some point or another just because of the extensive amount of episodes and seasons they had to produce. If you look at the worst seasons of them (Arrow season 4, Supergirl season 5,The Flash seasons 7 and 9), they all come around or after the halfway point of their run, and that is a clear example of why it probably would have been best to cap each of the shows at five seasons, allowing the writers a chance to let them bow out with a strong arc worthy of the final season based on a story that hadn't already been exhausted.
Sometimes less can be more, and if this allowed the Arrowverse shows to maintain (or rediscover) the standard of their glory years, we would have been all for it.