Just to give a little more ammo for your argument, eqthomas....
I took a loan a couple of game months ago to make sure I had sufficient cash on hand for the staggering number of C-Checks I had coming do in the next 3 months. I took 95% of what I was allowed unsecured (just rounded down to the nearest even mil)
Doing so didn't change my credit rating. I even checked again a game-day later to be sure. I thought that was surprising because early in the game every loan I took dropped me 1-2 tiers.
Then I began losing money as I expected. Over the past 6 game weeks I've gone from a healthy profit to some pretty large losses as I pay for all my C-Checks. This was expected, but I figured surely my rating would fall.
Nope. Despite my past several weeks of performance showing some fairly large losses on both the profit and revenue sides (since so many planes are not only not making revenue they're costing money to fix), not only has my credit rating not gone down, the bank has continued increasing my credit limit.