Won't work.
The problem is not backbone. The problem is the last mile. And building a network to compete with the telecoms is expensive to build. With cable or telephone it averages about $1500/house. That figure is fairly constant across many cities. Microsoft's infamous cash reserves would not suffice to create a competing network out of thin air.
Secondly companies are pragmatic. If Comcast slows eBay's traffic, it would be suicide for eBay to retaliate by blocking Comcast outright. Because most Comcast customers are locked in to Comcast in the near future, they are not locked in to eBay. And if eBay voluntarily gave up a large share of its market, eBay would suddenly be faced with not being the market leader. This would get very ugly, very fast. And even in the unlikely event that eBay succeeded in forcing Comcast to change its bullying ways, eBay would have just opened the door for a competitor (eg Amazon) to take over the online auction business.
No, Google et al are worried because the telecom companies ARE left with the power in this relationship, and everyone knows it.
Cheers,
Ben