This is not so black and white. I suppose there could be a real threat of a side chain taking over if there is little to no perceived risk by the market to move from chain to chain, but if the market perceives little to no risk then maybe there is little to no risk. Savers will likely park their money in the safest chain, i.e. the chain with the highest hashrate and bitcoin could cease to be the dominant chain at some point. I don't see why it couldn't happen.
I agree this could happen but not the way you suggest.
For a side chain to "take over" it requires the whole network, nodes & miners, to come to a concensus that the side chain should become the main chain.
As you have mentionned, it that were to happen, it would be because the market perceives little to no risk. Risk #1 being the conservation of value of their stake. In reality, ONLY a 1:1 peg that preserves Bitcoin's scarcity & overall economic model can provide this security.
I have demonstrated in my previous post and some have come to an agreement that sidechains issuing a new native currency (no 1:1 peg) are effectively an altcoin piggybacking off Bitcoin's security. For that reason, the likelyhood that such a sidechain takes over is the same as any regular altcoin taking over.
Having said that, if a sidechain's feature becomes so obviously superior that the whole market wants it then it is much more likely this feature is implemented into the Bitcoin main chain, either as a soft fork OR a hardfork. This makes much more sense for every market participant than having them all switch to the sidechain. In that scenario, hard fork are still a very tricky proposition but with the help of experimentation and beta implementation within a sidechain, the risk is considerably mitigated and consensus much easier to obtain.