For me, the short answer is removing full/near full-party premades was necessary. The longer answer is that it was a bit of a 'scorched earth' solution that highlighted some of the inherent flaws of their matchmaking system and Frontlines modes.

Premades absolutely could and did ruin the 24-man modes for the other 16 players. Whether it was just a party with a plan of attack/balanced party comp created beforehand (some roulette parties struggle to even get 1 healer or more), a group communicating through VC or discord, or just a group of vets looking to stack their party to steamroll everyone to farm battle highs; it was an advantage over everybody else who solo queued. There's a reason why solo queue Feast and light party Feast are separate and have 2 different rankings. A premade's influence arguably is diluted in 72 man modes, but people underestimate and downplay the benefit of having even one party full of battle highs/fevers marking/mowing down targets and leading the charge into contested areas or objectives. In my opinion, the devs should have had a separate queue just for premade parties, where they would have to be matched against other premade parties for the match to pop. I've seen the argument against that method being along the lines of "then they wouldn't get matches for long periods of time, if ever-" then that's on them to disband and queue up normally like everybody else. The other, extreme option was to prevent premades from queuing up at all- but instead they just decided to cut them in half, somewhat effectively limiting their influence.

What made this decision less-palatable was that it came at the same time of them removing 24-man matches- the one place larger premades absolutely dominated. I'm not quite sure what their reasoning was (they never told us), but it might have had to do with the maps being basically largely unchanged in terms of size and objectives (aside from only 3 flags in Secure, sometimes no big ice (or only big ice) in Shatter, and like 1 less tomelith in Seize). Maybe it was because more people disliked the small matches than those who liked them? I know I avoided Secure 24-man modes like the plague at all costs.

But with the removal of the ability to make anything over a 4-person premade, what cemented my belief that most people who did so were just looking for any advantage by stacking parties beforehand came with Onsal Hakair a week or two later. This was the other problem with frontlines: Grand Company allegiance. People looking to stack their parties with experienced players got around the new restriction by all swapping to one Grand Company (Adders on primal) and proceed to win what I would say nearly 75% or more of the matches. The devs tried to hold on to their flawed little GC lore and keep it relevant for as long as they could, but how it blew-up in Onsal Hakair was all the confirmation they needed that anything that could let players choose who they played with in any significant number versus those who solo queue needed to go. And hence why we have forced freelancer and GC is little more than what color team you're on now.