@Plasma Treats Safe Exits as the Major Scaling Assurance.
Plasma views Ethereum scaling as a risk-sensitive viewpoint. Rather than making optimizations to ensure continuous execution, it believes the off-chain systems might fail (via censorship, downtime, data withholding, etc) and makes decisions based on that fact.
Execution occurs on Plasma child chains, and the ownership is never relinquished out of the control of the base layer. The money can never be lost since there are predetermined exit mechanisms that apply to Ethereum. Should the operator misbehave or the chain unavailable the system does not make attempts to force through. It shifts into recovery mode.
It is what makes exits, period of challenge and fraud proofs the essence of the architecture of Plasma, rather than a supporting feature. Implementation is conditional, the recovery of money is not. This tradeoff is obvious: weaker guarantees regarding liveness, in favor of stronger ones regarding ownership.
Scaling, as used by @Plasma , is defined as failure survivability. It is more important to be able to walk things away safely more than it is to deliver on the assurances of always-on execution, and Ethereum remains the final fall back in the event of a collapse of off-chain trust.
