He just said that to make Cooper think he thought the basement was a good idea. Not to ruffle any more feathers (as I say this, I just watched like 5 conure bird videos on YouTube). But if you remember, he actually thought boarding up the windows on the ground floor would work and said very loudly and proudly that the cellar is a deathtrap, period - not "if we go down too early, it'll be a mistake." That's 1. Now, here's 2: if they'd been in the cellar earlier, they would have been paying more attention to Karen being sick and would have noticed her turning into a zombie. You clearly have your head stuck in the remake and it'll be pretty sad if I have to tell you that Romero's zombies do not in fact leap, run, or tackle people at any point in his films. So, believe me, there would have been time for the parents to decide something had to be done. This fantasy you have of the people in the house being lost in their thoughts and Karen being a super-zombie... yeah, no. Sure, there might have been conflict but even if Mr. Cooper said "nobody touch her," like I said- 90% of the people in the house are not at risk of being bitten. Not only does she not have super strength, but she will not gain super hiding abilities either. If they went down into the basement earlier... the power would still have been on.