Can't copyright gameplay mechanics, and the plot is basically "get a team of misfits together to battle an incoming evil", ie. it's Seven Samurai IN SPAAAAACE. Plus, one of the big criticisms that I saw about ME:A is that it's lazy; it's the same old aliens as we've seen before even though that makes no sense in a new galaxy, and the plot is just trotting out some weary old tropes. Very risk averse, very insipid.

So yeah:

• could spend millions getting the IP off of the shitty owners, to make a very safe and boring addition of the ME games

• make a completely new game with similar mechanics, but take some risks with the story. A fun game with a stupid plot is still fun; a fun game with a great story is exceptional. So why not roll the dice? You can get the staff easily enough to form a new company.