The Tesla truck will almost certainly beat them to mass market. Look how long it took Tesla to scale up production -- the Model S shipped in 2013. It wasn't until 2018 that Tesla built their 200,000th vehicle. Rivian hasn't yet shipped a single truck to customers; they're in for a long and difficult road ahead building out their production lines.
All that said I do hope Rivian is successful. I would absolutely buy an electric pickup truck.