You're right that it's possible in theory. However, we're talking about quite a few major changes that would have to be done.
1) Transactions are not aware of the blockchain at all. For this, they'd at least need to be made aware of the timestamp of the block they were put in. All such blockchain-aware transactions would need to be locked for at least 100 blocks after they are included in the chain before being spendable again. Even that might not be enough, in theory, although we'd likely be able to get away with it in practice.
2) Transactions would need to be aware of the output of the transaction in which they are redeemed. This is needed to find out how much is being spent and put conditional checks on it.
Neither of these are easy tasks. They might not ever be safe, either.
I'm completely willing to believe that implementing an "aware" blockchain is not easy. I also understand that awareness would lead to attack methods which try to manipulate awareness. However, I'm confident that awareness would open the door for a number of useful features (spending limits, blockchain-based derivatives, and others). I also believe that potential attack problems could be worked out eventually. I don't want to get into a discussion about how the features would work here. Instead what I am asking for is a long-term position on awareness.
It is a long-term goal of the development team to make the bitcoin blockchain aware?