Why rely on biogas digestion of the waste stream for that solution?

Because biogas and natural gas can run in the same very efficient, low cost distributed cogeneration device.  Namely solid oxide fuel cell/turbine generators.  They are scalable from backup for a home to powering a plugin hybrid to a 100 mw power plant.

Providing distributed backup generation for a renewable smart grid at very high efficiency (75%) and allowing cogenerated waste heat to be used where it is needed.

This allows a transition from natural gas to biogas as the grid goes renewable.  It also saves a lot on grid transmission capacity.  At first natural gas would be the main fuel, then as renewables and conservation start to take over, biogas supply would increase and natural gas could be mainly a backup fuel.

The cogeneration would help the process along, instead of extending the life of fossil fueled power generation.

The beauty of biogas from waste is it's offsetting characteristic.  Since the methane that constitutes the fuel part of biogas is 21 times worse as a GHG than CO2, if 5% of our energy came from biogas, that would offset the rest of our CO2 emissions.

That methane would normally be released from manure run off, sewage, garbage, and crop waste biomass directly into the atmosphere.  If we can intercept it, with biodigestion,  and use it to generate clean kwh, reduction in total GHG effect could push our carbon footprint to zero.

The power grid backed up by biogas could eventually be mainly renewable and we could actually go carbon negative, reversing the GHG effects of these industrial revolution and chemical farming decades.  

Biogas is the key technology, luckily it is well understood and in wide, but so far sparse application.  With targeted subsidy a boom could get biogas/fuel cell power to 5% of our power generation within a decade.