It's easy to convert gammas; the worst you get is banding when if you lose resolution and fa r movies this can be done offline, but when two color spaces don't align you'll have to truncate to the intersection where converting making the lager gammuts pointless. So given that nearly all big movies have been filmed and color corrected in P3 for years now, it makes sense to match their gamut, but match common display gammas, so that common sRGB content needs little conversion to be displayed.