I agree with this interpretation. However, I disagree that it won't distract from other stories. If anything, it guarantees that the third (and final) movie's climax will centre on this particular plot as opposed to any other plot.
Of course, it bears mentioning that those two movies then spend the rest of its time showing the fallout of those deaths. It is this dedicated follow through that is needed in order for the death to leave an impact on the viewer. If no consequences are shown for the duration following the death, said death would feel inconsequential. Sure, the audience will feel bad/sad as they watch the scene but five minutes later, they'd probably have forgotten about it.
Yes, doing this would leave the sequel less congested plot-wise than TASM. However, it still results in the third movie to be even more congested; as it would have to deal with (the entirety of) the aftermath of Gwen's death
and the plotline regarding Peter's parents. As the latter is the major focus and point of the trilogy, the former won't take centre stage in the finale over the latter.
Sharing focus during the third movie's climax would only weaken both subplots as each one would take attention away from the other. Likewise, conflating the two plotlines together into one (e.g. the person responsible for what happened to Peter's parents is the same person responsible for killing Gwen) also weakens the emotional strength and significance of each individual event as it muddies up his motivations. For instance, whenever Peter does an action (e.g. take revenge against this villain), the following thought process occurs:
1. Did Peter do this because of what happened to his parents, because Gwen was killed, or both?
2. If Gwen hadn't died, would what happened to his parents be enough to drive him to this action? (or vice versa)
3. If that answer is yes, does that render Gwen's death meaningless? (or vice versa)
4. If that answer is no, does that mean that Peter's feelings for his parents was not great enough to push him to this? (or vice versa)
The only way for it to work pacing-wise in the third installment is for one to turn into another (e.g. Peter's parents turn out to be alive and they (or one of them) is the one who kills Gwen; resulting in Peter's angst over their disappearance to evolve/be replaced by angst and guilt over Gwen's death).
Therefore, I contend that if there was ever a time to kill Gwen off in this trilogy (which seems to be focusing on Peter's parents' disappearance), it would have been at the climax of the first movie - with the second movie focusing on the fallout of her death and leaving the finale movie free to focus on Peter's parents - and that the opportunity (or at least, the optimal time) to kill her has now passed. Killing Gwen off early in the second movie is strategically and narratively unwise as it would reek of
Sudden Sequel Death Syndrome and threaten to break the audience/fan base. Killing Gwen late in the second movie results in the third having too many plots to focus on; as would killing her then.
Now, if there was a fourth movie or a second trilogy, then that would be an entirely different equation.