The parking at the stadium will be incredibly chaotic and is setting up to be a scandal. If this were perfectly planned it would be tough because it is new and anything new is destined to be chaotic. ( Remember the race track on the first race and how I-15 was closed. That was after some intense planning way beyond anything that's been done for this )
Nobody can know what the cab situation will be like at the game. But you can use the Airport as an example. McCarran has one of the best organized taxi pick-up systems in the U.S. and they are only capable of loading at most 1200 cabs an hour. They have a multi-million dollar staging area to bring hundreds of cabs close in to the loading zone. They also have a loading area that can load dozes of cabs at a time. Allegiant will never be able to move that many cabs without a large cab staging area and an engineered pickup system that loads dozens like this and they don't have it. Look at the map below and find the taxi area. You'll see there is no cab staging area - that means cabs will be on their own trying to wait for stadium rides. A potential nightmare. If Allegiant can move even three hundred cabs an hour they will be doing fantastic and that's still less than 1000 fans an hour. You are making a mistake if you count on either a cab or Uber.
IMO, plan to park somewhere on the strip and walk a couple of miles to and from a game. Or, hopefully, businesses nearer the stadium will open up their lots for a reasonable - $50??? - fee. Or see below for parking at one of four residential hotels nearer the stadium.
Going to the game should be nowhere near the issue that leaving the game will be. Arriving fans will be spread out over several hours. But departing fans will all leave at once. This is guaranteed to be a nightmare.
Taxi and Uber parking/pickup description from Allegiant website.
"The designated general and taxi drop-off and pick-up zone is located at the corner of Diablo Dr. and Procyon St.. Traffic will be directed eastbound on Diablo Dr. and northbound on Procyon Ave. There will be a walking path through Diablo Dr. leading towards Allegiant Stadium’s southwest entry. If available, a Taxi pick-up line will also be located in this zone after events. Allegiant Stadium has multiple designated Uber/Rideshare areas, conveniently located within a short walking distance of the Stadium. Guest rideshare drop-off areas will be based on ride start location."
POSSIBLE RESIDENTIAL HOTEL PARKING. Looking at the map, find the South Ride-share area. ( Oquendo and Century Park Drive) There are four hotels with loads of parking. ( Fairfield, Courtyard, Residence and Holiday inn) I would never try this, however, without first calling the hotels and seeing their game day parking policy.
Anticipate this will be a total sh*t show. Plan ahead. Call ahead. Stay calm. Avoid confrontations. (Nobody needs to get hurt over a parking spot)