# Lots of transactions. That would make the cost of running a node capable of validating new transactions expensive. Gone would be the days when your laptop had the capacity to check a transaction for double spending. Also, it would take a long while for your transaction go get into a block if there were thousands of new transactions each second flooding the network. If you combine that with large mining operations that would build blocks out of just a small subset of the available transactions and win most of the races for the next block, that would mean that making your legitimate payment could take a long time and the system would feel unreliable to it's users.
From my limited understanding of the bitcoin protocol, the transaction fees are designed to solve exactly the above problem. When the volume of transactions becomes problematic, then miners will be free to choose which transactions they include in their newly discovered block, and the prioritization will be done based on the fee included with each transaction. In this way transactions with no fees can be treated as spam and ignored (or will take very long time to confirm), and only transactions with sufficiently high fees will be included.
Regarding your other points, I wholeheartedly agree, especially with regard to modularizing the code with clearly defined security boundaries between each module. Having a gigantic monolithic piece of code where a single vulnerability may bring down the whole thing is bad software design.