If the aftermarket firmware doesn't support Netflix, then users will have to choose between not having Netflix and having the features the aftermarket firmware enables.

If that aftermarket firmware enables better playback of pirated content, and the Netflix catalog deteriorates more and more with time, it's more and more likely that of the people who'd consider alternative firmware in the first place, more and more will decide against Netflix in that scenario.

Not that that may be enough people for Netflix to care, but it could easily be enough that "no netflix" is no longer a blocker for acceptance of the alternative firmware.