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.
now you're rolling.
this is why i call it dilutional to Bitcoins current core function and value; that of money.
not only that, a SC for stock would introduce a time preference demand into the equation. whereas BTC currency is supposed to be highly liquid and hold value, a stock is a long term investment and introduces risk which negates Bitcoins money function in aggregate. i would argue that the non US world could care less about Bitcoin as a vehicle to invest in stocks, bonds, etc. they only care about the SOV money function. Americans only care about this too for the most part if you look at the stats (relatively low participation).
if we start diverting our attention away from the core problem for which Satoshi built Bitcoin, that as money, it just becomes another WoW trading platform game.