Bottom Line Over Architectural Philosophy
If all other things are equal, there are clearly cost advantages for vendors delivering multi-tenant SaaS applications versus single instances of apps run in a vendor's cloud. But the bottom line is the cost to you, the customer, and we can't assume that all other things are equal.
A SaaS player, for example, might have high development costs and a small scale of economy, whereas an incumbent with lots of customers can quickly scale up hosting/managed services without doing much of anything to its software. You have to answer the questions about agility and cost, but if the flexibility and speed are good enough and if the costs are right, then don't get hung up on architectures.
In the long run, Darwinian forces will see the superior approaches prevail, but that process of natural selection may take longer than your committment to any one cloud app.