Because the prior version _already_ faulted in production as a result of having a trust graph that didn't meet the prior criteria needed for safety. The prior version also resulted in centralization in practice (in it's ripple instantiation; kind of neat that the ripple->str-reboot let us see both of the predicted failure modes play out, even though they were largely mutually exclusive)
The latest work is intended to relax the requirements/consequences but still provides no guidance or tools for achieving the required topology in practice.
By all means, clearly label these efforts that have have taking a "you haven't yet proved its broken" approach to safety/security; and I won't complain about them. Absent that, I just normally expect that when someone produces a cryptographic product that they've actually given some care to their security.
> consequence is that people will start putting the Stellar equivalent of blockchain.info onto their trust list in order
Then if they're anything like the Bitcoin world's blockchain.info-- which is regularly in a confused state--, I'm may soon find myself the proud owner of infinity STR, I guess? ( Screenshot someone else sent me of the actual BC.i site one day a while back: http://people.xiph.org/~greg/21mbtc.png )
"testing is making sure it does what it should, security auditing is making sure that is all it does"
In SCP the topology is public and conveyed with each consensus packet. So people will be able to tell when the graph is vulnerable.
Improving the definition of topology requirements for correct consensus is, far from being 'ignored', exactly what Mazieres has been working on all this time. And as you admit, those requirements have now been formalized and the information to check them is conveyed in the consensus packets; they are just not trivial to check by hand, and we have not yet implemented a check for them in stellar-core (this will be forthcoming, see roadmap).
I'm also interested in how the explicit quorum slice data for each node can be used to maintain quorum intersection over the entire network as new nodes join.
That said, my _prediction_ is that it will work fine mostly, occasionally there will be concerns about people splitting off into islands, and a resulting second-order consequence is that people will start putting the Stellar equivalent of blockchain.info onto their trust list in order to ensure connectivity to the "main graph" (I had actually cited The Tyranny of Structurelessness in my own responses already :) ), and this will just have to be the social-network-consensus version of the GHash.io scare and we'll be fighting against people's private interests to be lazy to reduce the risk of that happening.