Why Flashpoint Shouldn't Reboot the DCEU
As it stands, the DCEU actually has a lot going for it. While there have been some missteps, Warner Bros. has packed a lot of story into its DC movies. The backstory of the DCEU rivals that of the MCU in size, and it was accomplished with far fewer films. The DCEU doesn't need to abandon the world that's been built - it just needs to use it more effectively.
Take Batman and the backstory that Batman V Superman: Dawn of Justice gave us. His origin was covered very briefly: we know he used to have a Robin and the Joker killed him, and that he's had years of adventures offscreen, but is still regarded as an urban legend. Justice League added more details, including the fact that Penguin also exists within the DCEU's past. These characters and events don't have to be erased or modernized to be effectively told. Future Batman stories could take place within the 15 years that he's been active, told either through flashbacks or dated at the beginning of the film.
This isn't to say that Flashpoint shouldn't have an effect on the DCEU. A reboot of the movie universe shouldn't be that effect, though. Little tweaks, maybe... but starting over with a "New 52" effect would kind of defeat the purpose of the story.
Rebooting Isn't the True Purpose of Flashpoint
In the comics, Flashpoint is widely considered one of the best Flash stories in recent years. The story arc had unexpected twists, it gave us glimpses of alternate versions of popular characters, and it had a lasting impact on the comic universe. Some of its effects are still unwinding in the comics today. At its core, though, it wasn't just a way to reboot DC into the New 52. It was also a really good story - and, perhaps more importantly, a really good Flash story.
This last point is perhaps the most important. Flash Comics #1 appeared in late 1939 (cover date January 1940), just after the debut of Batman and a year after Superman hit the scene. The Flash as a character actually predates Wonder Woman by almost two years, and the Barry Allen version of the character has been with us since 1956. Despite this long history, the Flash is not part of DC's "holy trinity." There are relatively few Flash stories that are real stand-out moments in the comics, especially compared to Batman and Superman. Flashpoint is definitely a high point for the character, and is a great story to tell given what we know of him in the DCEU so far.
The true purpose of Flashpoint is to take a character who is more powerful than many (including himself) think and show that there are some problems that power alone can't fix. Barry Allen's attempts to fix things only make things worse. His actions in Flashpointseem like something that the DCEU Barry Allen would try, and the fallout of that is necessary if Barry is going to grow as a hero. Justice League was the first step in him realizing his destiny as a hero. Learning his limitations in Flashpoint will be the next step.
A DCEU Reboot Wouldn't Work This Way
While Flashpoint seems like a good point to reboot the DCEU, it wouldn't fit in with the way the DCEU has worked so far. Superman set off the DCEU as we know it, and his death "rang the bell" that set up future conflicts. The defeat of Steppenwolf threw down the gauntlet showing that Earth shouldn't be messed with. Rebooting the continuity as an aftereffect of Flashpoint seems somehow anticlimactic after all of that.
On top of that, the DCEU has a much better source to draw from if it really needs a reboot: "Crisis on Infinite Earths" (and the other "Crisis" events). "Crisis" was designed specifically to simplify DC's past storylines, so it would work well as a reboot film event. It's sufficiently massive to fit the escalation in the DCEU narrative thus far. It also gives a perfect setup to trim off bits of the current DCEU that don't really fit. Perhaps most importantly, it's a big enough story that Warner Bros. could make a two-film "event" like was originally suggested for Justice League without it seeming out of place to do so.
Whether the DCEU sees some form of a reboot with Flashpoint remains to be seen. If we're lucky, though, the DCEU will save any reboots it needs for a much more appropriate film.