What occurs in practice should pale in comparison to what happens in live action in a game setting.
Reducing the games down to two basically gives teams one game to really see their backups with extended playing time. It's simply asinine to expect teams to make judgment calls on players off of one game.
It's a horrible idea. There isn't even a debate to it.
This just shows how little you know.
They are getting hundreds to thousands of reps in practice to show the coaches what they can do. In the preseason, with the enormous size of the rosters, they are getting maybe 30 to 50 plays per game to evaluate the player. A good preseason performance might give the edge to a player who is already in a close race with another player, but it is not going to be used as some sort of huge evaluating tool. The vast majority of the coaches evaluations are done outside of those preseason games, which again is why we see preseason heroes cut year after year.
The biggest thing cutting the preseason short would do IMO would be to make teams look alot more rusty in those first few games, at least the first couple years of it. Aside from that it is not that big a deal to lose those two games. It's more about team preparation than player evaluation.