a single Blockchain is a single point of failure.
The problem from an investment perspective is that this requires active asset management, and if I choose the wrong chain to allocate some of my wealth to I lose when it fails (assuming the 2wp is also lost). I like spin-offs because if you have a 1% stake in the ledger now, you have a 1% stake in all the spin-offs as well (assuming spin-offs have the same inflation schedule as Bitcoin) by default, without having to do anything. No matter what chains win, your stake in the ledger is always and automatically preserved. With sidechains, you have to be a masterful investor to ensure net zero change to your stake in the total operational ledger.
But spin-off create a different ledger no?
Once transactions occur on the spun-off chain they don't consolidate on the other chain
Yeah, that's true, but as a hodler you can just hodl even if others choose to transact. To participate in transactions (i.e., to alter your percentage of the ledger) on any spin-off is opt-in, rather than opt-out.
Suppose I hold X% of all the BTC now. If I never spend on, say, a Monero spin-off of Bitcoin using a snapshot of the Bitcoin ledger today, my stake remains X% in perpetuity. If the Monero spin-off one day takes over, I still control X% of it no matter what other holders have done, so my position in "the ledger" (what is currently known as "the Bitcoin ledger") is preserved by default even if I was frozen in ice the whole time.