Actually the reason why publishers focus on the initial sales is because it is harder to bank on long-term returns. So to them the amount of units sold on Day 1, Week 1 and Month 1, in that order, is the most important because it recoups the initial investment for making the game in the first place. So if a game sells 500.000 units in a single day, they get better projections as to how many could be selling in a week or a month or a year.
Course that all loops back on failure to properly budget because publishers arent interested in recouping their investment, they want to make mad bank. If a game merely recoups its budget, lets say 20 million, it isnt good enough because the game failed to make profit, to them it is only successful if it gets 21 million or more because thats 1 million in profit right there.
So it makes sense why Square Enix called the reboot a failure in that sense cause..well it didnt make mad profits right away, which is why the industry is in such a shitty state.