Red-based Prowess has an interesting history in Modern because the deck has existed for a number of years, but the current builds barely resemble the way the deck was built many years ago. Past Monastery Swiftspear, Lightning Bolt, and Manamorphose, almost all the staple cards are fairly recent by the standards of the format. The additions have been less surgical tools and more raw rate and redundancy, and the deck has risen in representation and win rate accordingly.
Another credible one-drop beyond Swiftspear, text box meaningful for a strategy that can be soft to large creatures.
Low-opportunity cost cycler (one mana, doesn’t require a target), text box comes up in creature matchups, or anyone trying to buy time by chump blocking.
Absolutely busted in this deck along with Mishra’s Bauble (and other cards) pre-companion nerf, still a strong incentive to explore white as a secondary color.
The initial Modern Horizons pickups. The lands are strong even if you aren’t playing a second color. Lava Dart is absurd in almost all the creature matchups and is good enough alongside Prowess creatures that it can carry its weight even against control or combo strategies.
Card advantage, velocity, staying power in your sideboard games. Expressive Iteration is outrageous; anyone arguing that Light Up the Stage is better in absolute terms should be ignored, but it's still plenty good to play if you aren’t playing blue, or if you want a fifth or sixth copy of such things.
Iteration was the inflection point where blue became the default color instead of white. Before Modern Horizons 2, a stock list looked something like:
I played this build, and builds similar to it, and I was not impressed. There are all sorts of inconsistencies engendered on the margins — lines that require two blue mana among your first three lands, having to fire off spells at inopportune times to cast Stormwing Entity, and a bunch of dead weight against control such that beating something as innocuous as a removal spell and a Snapcaster Mage was very difficult. I eventually moved the situational stuff (Snag and Growth) out for Lava Spike, which raised the floor and lowered the ceiling. My sideboard eventually “evolved” to include cards like Snapcaster Mage and Threads of Disloyalty, an indictment on how unwieldy the deck was, particularly against resistance.
That brings us to Modern Horizons 2 and Dragon’s Rage Channeler. I believe this is a massive addition and has played out that way so far. Besides being absurd on rate and in the context of the deck, it's another powerful, no-brainer red card. We’re now in the territory where there’s no need to play anything sketchy or situational, like mopey blue threats or Abbot of Keral Keep to fill out for Lurrus builds.