Yes, you are correct that remaining backstop in further draws is available under the SPSPA. However, it costs the government around $250 B per year to continue this credit facility. THAT is a lot of budgetary pressure on revenue and both the debt ceiling and the ballooning government deficit. From the government's perspective, the risks of maintaining the backstop while the GSEs rebuild capital may be far greater than just putting them in the Waste Management corporate dumpster along with General Motors.
I HATE the thought that this could happen. I think it could be a travesty. But I encourage you to consider its possibility before simply dismissing it.