Every character had a motivation, even if it was as simple as "Empire = bad", they may not have been particularly good or fleshed out motivations, but they were there.
I'd argue that if you were fully invested in every character and then they all started dying off one by one, each of them with the full weight of emotion behind it, it would have felt forced after the 3rd or 4th death. Like Dray already mentioned, it has a very war movie feel, character dies, and we move on. I felt something for every death bar three (the Pilot who I honestly didn't even realise he'd died it was over that fast and Jyn/Cassian because I thought they were both quite bland characters).I have read that excuse of "plot driven, not character driven" before, and I dont think its good. You cant have a good movie without a bit of both. The plot should move the characters and the characters should move the plot. The fact they die means maybe they should have given them more weight. It brings more gravity to their sacrifices and without changing the scenes themselves, makes the scenes more impact.
I'd also argue that in a film where you know the entire main cast is going to die, developing them is simply a waste of screentime.