So, we've been told multiple times by characters in the series that the reason why the Goddesses attacked the Demons upon Meliodas defection was due to the power balance being in their favour and as such, for Gowther to end the war, he had to restore this balance by removing an equally as powerful individual from the Goddesses as Meliodas. Apparently, this had the desired effect given how the Goddesses were forced to activate the seal upon the balance being restored, but logically, couldn't they have won without doing so, even with Mael's ''death''?
Here's how I see it:
-Even with Mael's absence, the Goddesses were in a favourable position against the Demons. Supposedly, they still had every other AA, with the addition of Elizabeth & Meliodas. If Elizabeth or Ludoshel especially, were seen as Mael's equal, why didn't they step up and take his place? There's an emphasis told on Mael's power, yet it's revealed he has people who can equal or surpass him, weakening his importance.
-Ideally, the war could've been won for the Goddesses. In my opinion, the Supreme Deity was foolish in deciding to team up with the Demon King, rather than using Meliodas & Elizabeth as tools of war. At this time, Elizabeth & Meliodas were helpless before her and it's been revealed Goddesses have very capable brainwashing powers. What was stopping her from brainwashing them into doing her bidding? Unless the Demon King intervened, there's nothing which would interrupt this plan.
-Not only this, but the Supreme Deity didn't even try to locate the sun grace and bestow it upon a new user, such as Ludoshel or brainwashed Elizabeth. Why? The Demon King gave Zeldris representative powers to manually recall the commandments from a large distance and even incorporated an element of communication with the commandments, so he could talk to them about what the user was doing. Assuming the Graces are similar in function, what was stopping the Supreme Deity from finding the sun grace? In fact, the whole dilemma of ''Mael's death'' could've been undone quickly if it's assumed Gowther's brainwashing doesn't work on non-living entities (such as the sun grace, whom was with Mael when he was ''killed''). The Supreme Deity taking little action to remedy the death of her strongest AA is silly, especially for someone whom is reversed as a ''God'' and a military leader of a very powerful clan.
-Assuming the Supreme Deity did at minimum, find the sun grace & brainwash Elizabeth/Meliodas into fighting against the Demon Clan. They wouldn't be required to activate the seal since Mael's position would've been replaced by a Sun-Ludoshel/Elizabeth. Not only this, but during this point of the war, the strongest fighter on the Demons side (besides the King) would've been Zeldris, since Meliodas betrayed them & Merlin mentioned how the Masters didn't participate in the holy war. Whilst Zeldris is strong, I'm pretty sure any of the AA's & Mel could take him alone, leaving only the commandments left, whom could also be handled easily by one or two AA-tier fighters. Even with the addition of the masters (or OG Demon) & single-Indura of favour (that was recently revealed to be fodder), who could stop a Sun Grace AA with other AA tier fighters as well?
In essence:
-Supreme Deity handles the Demon King
-Sun Ludoshel, Elizabeth or Meliodas could individually handle Zeldris & Masters/Indura with some difficulty.
-Sariel & Tarmiel could wipe out the entire commandment brigade, assuming Zeldris wasn't with them.
-After wiping out the respective threats, the AA tier fighters could just finish off the rest of the Demon Clan fodder, and then hypothetically come back to help the SD against the DK.
This is entirely hypothetical, but didn't the Supreme Deity/Goddesses still have a favourable position against the Demons had she acted instead of killing more of her strongest soldiers & committing suicide?
TL.DR - SD should've brainwashed Mel/Eliz, find & give the sun grace to Ludoshel and handle the DK whilst Eliz/Ludo/Mel/Sar/Tarmiel wipe out Zel/Masters & Commandments. They could've won and suicide was silly.