No matter how strong bitcoin is, I think it is VERY dangerous to put all your eggs in one basket.
That said, I think side tree's are a great idea for Bitcoin, but should not be the only game in town.
The more decentralized the overall structure, the more powerful imo it will be.
A spin-off is different than a side-chain or a tree-chain because the payment network for the spin-off is completely independent of bitcoin. The spin-off pre-mine only preserves the distribution of wealth encoded in the Blockchain at a certain Block #. The spin-off is bootstrapped such that bitcoin holders can claim their pre-mine from Block 0 by producing a signed message with their bitcoin private keys. Once the spin-off launches, it becomes entirely untethered from bitcoin, evolving as nature sees fit. This adds redundancy without wealth redistribution.