Hi! I can totally understand the philosophy of having the entire game unlocked from the beginning, but this is something I've added after playtesting with friends. In roguelikes especially, players want to feel like they are progressing, even after a lost run. Another point of the progression system is to increase complexity over time. Some playtesters were jarred by the number of units available from the start. They felt like they were spending too much time in the new game menu studying upgrade paths and starting items for all 16 unit presets in their first run.
But the unlock speed scales insanely with difficulty and enables skilled players to unlock the desired content in no time.