Mobilicity only provides urban service, so they only put up towers in areas with high concentrations of potential customers. Shaw would be expected to cover rural areas too. A better comparison would be Wind, who plan on eventually offering rural coverage as well.
I think the problem Shaw faced was that they were so disastrously late to this party. Shaw took their sweet time sorting this out, and by now Mobilicity and Wind are firmly entrenched in Calgary, Edmonton, and Vancouver. If Shaw were to build their network now, they would have to offer similar rates to those two, as they'd be offering similar handsets and their network would almost certainly be weak to start. I think they'd have difficulty attracting customers, and they'd have razor thin margins on the ones they did attract.
Unfortunately for Shaw, they don't face the same competitive environment as Eastlink, for example, who currently have no new entrants operating in their market and only anticipate the arrival of Wind, so they can afford to take their sweet time.
The wifi network idea is interesting. If they can build a solid network, they can offer data only devices, which bring the highest margins anyway, like tablets and laptops. The spectrum is free, and I'd bet the equipment is cheaper too.