I appreciate being able to build a decklist using cards that aren't in my personal collection from the archive, and being able to save and build the deck card by card is a nice thing to have, but being unable to playtest the deck until it is done can be a huge waste of time and resources (namely wild cards, which are hard enough to come by as a free to play player) if the deck turns out to not be that great.

Being able to playtest a deck vs a bot, or even in Direct Challenge scenarios against friends, using unrestricted access to the entire catalog of MTG Arena's cards, would be a huge help for deckbuilding, and would drive sales of wild cards up. If, for example, I am able to build a deck of cards that is entirely of cards that I do not own, and I playtest it against bots, or against a buddy of mine... and it works? I'm 100% more likely to buy the wildcards I need to finish the deck, and that is purely because I was able to try it out first.

There would be no need to change the reward system to compensate. The bots are really only good for those who are just beginning to play MTG or are trying to test something new anyway. You don't get daily rewards, or achievement progress, or win rewards for playing them. You don't need that, either. Just a better way to playtest.

Imagine going up against a deck you have never seen before (a wacky Mizzix's Mastery deck, for example), saying "man I wish I could try that out", looking up a decklist using what cards you want and can use, putting it together, and it works just the way you saw someone use it against you. It's the same as borrowing your buddy's physical deck, trying it against them, and saying "now I want this deck". You're infinitely more likely to buy a deck you're able to try for yourself first. It works for cars, clothes, food, why would it not work in a trading card game? It seems like a no-brainer decision to me.