To no fault of the Battlefield series as a whole, the mode simply fell flat due to its lack of evolution/innovation in a packed genre filled with different takes on the core mechanics. Part of that was Firestorm shared a lot of the problems that Battlefield 5 had overall as well. Regardless, the mode fell by the wayside as other core Battlefield modes had continued support. Now Battlefield 6 is aiming for a 2021 release, and another version of Firestorm may return. If that's true, the series will have plenty of contenders to battle for battle royale supremacy in the next-generation.
Even if Battlefield 5 as a whole lost steam pretty quickly, Firestorm became twice as irrelevant regardless. While Firestorm did receive some initial praise critically, future updates didn't make any significant changes to the battle royale's balance or content. Generally the opinion of Battlefield fans was that they honestly just didn't want a battle royale from Battlefield 5, and none of the loot or the mechanics helped sway that opinion. The worst was when Battlefield 5 updated the time-t0-kill across all multiplayer modes, which apparently was never properly implemented in Firestorm. With the increased health and rebalanced weapon stats for multiplayer, the changes spilled into Firestorm presumably without testing. Players nearly became impossible to kill with Firestorm's different health stats compared to regular multiplayer.
As a result, Battlefield 5's Firestorm became unplayable for many fans of the mode, effectively killing any interest in Battlefield's attempt at battle royale. So many other battle royales just offered more enjoyable versions of the same core experience, so players abandoned the experience. Though partially due to the issues with Battlefield 5's core multiplayer modes, Firestorm also seemed to lose support from the developers as they shifted focus to fixing other modes. Lack of support from both ends basically let Firestorm die off without so much as a whimper, which is a shame because there were certainly good bones in its structure despite its many shortcomings.
This shouldn't discourage DICE from pursuing a battle royale in the future, and with another Battlefield game confirmed for 2021, it'd be worth another shot. That being said, assuming there is another Battlefield battle royale in the pipeline, there's going to be some stiff competition for future battle royales. Mainstays like Fortnite and Apex Legends have still stuck around to this day, evolving and changing with consistent support and dedicated fanbases. Even Call of Duty has proved with Warzone that established franchises can succeed in the genre as well, so long as the innovations is there.
Battlefield has the capability of generating a successful battle royale game, but the support and care has to be there. Additionally, the franchise needs to bring something new or unique to the table to encourage players to opt-in. Where many aspects that made Battlefield unique before are now core portions of most battle royale games, there's still signature aspects of Battlefield that DICE can take advantage of. Regardless, if fans lost a little faith with Firestorm's failure, there's absolutely a chance for the next battle royale mode to be better.
To do this, Battlefield needs to capitalize on what makes it special, come up with a unique and defining feature, or even both. One major thing that Battlefield still has over every other shooter out there is the sheer destruction the Frostbite engine is capable of. Destructible environments have been a mainstay for the franchise since Bad Company 2, but it's become such a non-factor in recent entries. Emphasizing how destruction can change Battlefield's battle royale would be a perfect integration of core mechanics with things like mortar strikes, more vehicles, anything to dial in on the chaos Battlefield does best.
Lastly there's that secret sauce to differentiate another Battlefield battle royale from its competitors, which is something very hard to narrow down or articulate. Warzone has the Gulag to serve as a player's second chance in each match, Fortnite has its iconic building systems as a core gameplay mechanic, and Apex Legends has the hero abilities that players can utilize. Firestorm never really had anything special like that, in many ways it just emulated the base battle royale structure. Battlefield's next battle royale needs a unique mechanic(s) like this to prove its difference in an enticing way.
At the end of the day, Battlefield would benefit on giving the battle royale genre another shot. Assuming the support structure is there, plenty of lasting content is planned, and there's enough care put into the battle royale's beginning days, then there's no reason a Battlefield battle royale can't succeed or carve its own niche in the genre. Hopefully Firestorm and Battlefield V doesn't discourage DICE from giving the evergreen shooter genre another try.
Battlefield 6 is reportedly in development for 2021.