A Beslan style attack would harm al qaeda in the short run. Preparations would be noticed. Moving even 100 arab fighters into the US would set off alarm bells in the intelligence establishment. No matter how bad the southern border is, large-scale movement of foreign fighters into the US would be noticed.
Also, it would only serve to wake up the American public. Americans would remember that they are directly in the crosshairs of the islamists and would regain their resolve.
Loss of Pakistan would give al qaeda another safe base of operations and would be rightly perceived as a major loss for the US.
If al qaeda strikes in the continental US in the future, they have to top 9/11, not just anger the US populace. They need a country where they can operate with impunity in order to plan a massive operation.
They will surely succeed unless a miracle intervenes. Most are just not vigilant enough. Someone once said that the price of liberty is eternal vigilance.
Looks like not enough people are paying their dues.