WARNING: The following contains minor spoilers for the Demon Slayer: Kimetsu no Yaiba manga by Koyoharu Gotouge, available in English through Viz Media.
With the confirmation that Season 3 of Demon Slayer: Kimetsu no Yaiba will adapt the manga's "Swordsmith Village" arc, fans of Zenitsu Agatsuma and Inosuke Hashibira are in for a letdown. Zenitsu and Inosuke each had breakout performances in the "Entertainment District" arc, making vital contributions to the fight against Gyutaro and Daki. This is why it's extra disappointing that Tanjiro Kamado's travel companions and closest friends will be absent for the majority of Season 3. In Demon Slayer's next storyline, Zenitsu and Inosuke do not travel to the arc's titular village with Tanjiro, meaning fans of the anime have seen the last of them for a long, long time.
At the time of writing, Season 3 has not been given a release date or even a vague estimate of when the anime will return. Two years passed between Season 1 and Season 2 -- although that may have been in part due to production on the Mugen Train film -- so it could be a while before Demon Slayer's return. Judging by the manga, without getting into specifics, it probably wouldn't be until a hypothetical fourth season that Zenitsu and Inosuke would see significant screen time again. Unfortunately, this means that fans of the characters will likely have to wait several years before seeing the Thunder and Beast Breathing swordsmen again.
Demon Slayer's "Swordsmith Village" arc spends its time with other characters from the series, so Zenitsu and Inosuke were forced to the sidelines to avoid a harmful overload of characters. Barring any massive changes to the source material, Zenitsu's and Inosuke's presences in Season 3 will be minimal at best. A disappointment for sure, but it explains why Zenitsu and Inosuke went so hard in the battle against Gyutaro and Daki -- it was their last hurrah. They're each so severely injured anyway that they could certainly use a break, considering Zenitsu has two broken legs and Inosuke was stabbed through the chest.
In all honesty, the end of the "Entertainment District" arc is the best place to step away from Zenitsu and Inosuke for a while. Each of the fan-favorite characters experienced tremendous character growth in Season 2, so there's little about their respective stories that audiences will be waiting to follow up on when they return. Zenitsu has finally grown beyond his crippling fear of demons, as he brashly confronted Daki alone several times during the conflict. Inosuke has matured into a reliable, competent Demon Slayer whose disgusting abilities saved countless lives, including two of Tengen's wives, in Daki's food locker.
None of this is to say that Zenitsu and Inosuke are no longer interesting characters or relevant to the series. Only that now is the ideal time to focus on some other characters who deserve Season 3's spotlight. Demon Slayer is at a high risk of stagnating as it has settled into a formula of "new demon, new Hashira pal, kill the demon, recover at Butterfly Mansion, train." Sidelining two of the core characters in order to emphasize other supporting players is a helpful move to keep the series from getting repetitive. The amount of time that Zenitsu and Inosuke will be gone from the audience's perspective will also help the series feel fresh again for their eventual return down the line.
Tengen Uzui would not have survived the conflict with Upper-Rank Six without the help of Tanjiro, Nezuko, Zenitsu, and Inosuke. The cost of victory in the "Entertainment District" arc was high, so now is the perfect time for some characters to get some rest. Tanjiro and Nezuko are the central characters of the entire series, so they can't exactly be removed from the next storyline. While it's regrettable that Zenitsu and Inosuke drew the short straw in this regard, their performances in Season 2 were just epic enough to keep fans satisfied with them long enough to endure their upcoming absence from Demon Slayer.