Lake Las Vegas Pkwy is the main road into the community, and as a secondary road, it would be the last major road that Lake Mead Pkwy intersects before it reaches the gate into the Lake Mead NRA. An argument for it to be extended to the gate could be made, like how the US Routes do before entering Yellowstone, but I figure it would make more sense for it to terminate at the last major populated place and the intersection where most traffic would be splitting off.

I had the main roads through the Lake Mead Rec Area upgraded to primary in the past but downgraded them to secondary a couple years ago since they're realistically only used by traffic local to the park rather than by through traffic.

On a more unrelated note regarding the Lake Mead Boulevard section you upgraded to primary, I think it could make sense to leave a primary spur from Nellis Blvd to Hollywood Blvd, since it's the main E-W route through Sunrise Manor and has a much higher AADT than the rest of the secondary roads in the area and supports more commercial development as well. Though, I could see a possible primary connection along Hollywood between Lake Mead and Boulder Highway in the future once the new bridge over the wash opens up and thus creates a new eastern bypass for the valley.