It's a massive catch 22 and shows how poorly planned out this was. I can sympathise with their ultimate vision, in terms of franchise treatment. If you don't want to continue a franchise and yes, create a definitive 'end point' for the saga, it's nice to throw some fuel on the mystery pile to keep the long time fans speculating. That way anything that happens after can be in the realm of wonder and speculation, something big fans will ideally eat up.
Their big mistake was doing it with a franchise that is barely a franchise. There's only three mainline Mass Effect games, and all are part of the same trilogy arc. To make matters worse, this trilogy was supposed to have very specific resolution to multiple plot threads, many of those plot threads emphasising drawn out post-game consequences. They cut their franchise short too early, too quickly, when the franchise was arguably still growing, and did so in a means that chewed up and spat out the original plot threads that themselves fuelled speculation. They created new unprecedented and unfounded speculation for their legacy.
If ending the franchise this way was their intention, it would have been far more logical to end it in a way that still takes into account players' decisions. Would the Krogan florish under Wrex' rule, or would they conquer the galaxy? How long would the Krogan survive with Genophage remaining? With the Quarians and Geth united, would they be accepted into galactic society? If just the Geth, how would they respond? Will Thessia and Palaven be rebuilt? And so on.
They had already sewn the seeds for a future that could exist only in the minds of fans, best of all, in an imaginary state based on play style and decisions. Your future wouldn't be the same as mine, and though we'd never know, we could still speculate.
But nope. Let's disregard everything, shit on what we established, and throw out a bunch of new dumb mysteries for 'fans' to gobble up, with no intention of ever answering anything. What could possibly go wrong?