Without looking at specific players, and in normal seasons, I'd say your thinking is spot on.
We generally see Coaches chasing the hot player, paying too much for them too late, then bemoaning their return.
On specific players, I can see the potential for Simpkin to still be an OK trade in this week, but it might start to hit the questionable area after that.
Neale is an interesting one. I'm expecting him to go 150+ again this week, against Ade. HOWEVER, I'm expecting after about round 5 or 6, he might only average around 110 for the remaining games. As you alluded to, do you want to pay $750k, for a player that goes in simple terms 160, 140, 110, 110, 110, 110 etc. Yes 110 is good enough for an MID Keeper, but a 115 return on your money, is not good enough for a $760k player. At those numbers, he will very quickly be below $650k again.
I agree with your sentiment. Set reasonable expectations. Don't adjust them by large numbers, based on 2 or 3 games. Look at the return on your investment over the rest of the season. Try to look more than 1 game ahead!