Some BBS posters let their own strawman fallacies of Terry Matalas live rent free in their heads. Is the season perfect? No. But of the Star Trek we've gotten in the last decade plus, it's the most "of a piece" with the Star Trek that came before 2005.
A Borg / rogue Changeling alliance? It makes sense both from a production perspective and in-universe.
Not only were the Borg the Big Bad for TNG and VGR, but they are likely the most new-to-the-Berman-Era Trek species known to the general public. The Changelings were the Big Bad for DS9. PS3 was like a movie and a TNG reunion. You aren't going to have forgotten one off episode villains be the Big Bad. The Borg sell, and are understandable to the General Audience. Shape shifters are also easily understandable by the General Audience, many of whom likely have a distant memory of DS9 from when it was airing.
Contrary to rumors, Armus and the Pah-Wraiths just weren't going to be the main antagonists of a TNG reunion. Reunions are going to feature the Biggest Bads of the main characters' history coming back. The Dominion are the greatest threat from the Gamma Quadrant. The Rogue Changelings serve as a risk of reigniting the Dominion War. The Borg are the greatest threat from the Delta Quadrant. Just what happened to them after "Endgame" is a legit plot point to follow up on. The books did something similar, after all.
Is it a stretch that the rogue Changelings and the Borg crossed paths? Sure. But, again, the greatest powers from the Delta and Gamma Quadrants are going to be known to each other. The ship of the mystique of the Borg sailed with the introduction of the Borg Queen. Why wouldn't a crippled Borg Queen driven by vengeance not try and collaborate with a similarly scorned party who seemingly wants the same goals? The small universe syndrome card doesn't really work with the top major players. In real life Stalin and Hitler teamed up at first as well after all.
The target audience for PS3 was likely people with a past passing familiarity with the Berman Era Star Trek. PS3's continuity call backs are actually much easier than the ones from season 1 and 2.