I think we all agree that Bitcoin is good enough right now and actually pretty close to being perfect as is. As I have mentionned above, its only shortcomings are in its transportive properties : confirmation times and inability to provide private transactions natively. This leads me to believe these two aspects are likely the only ones that who will lead to the creation of utility sidechains that are used as subsets of Bitcoin-money. In all honesty, is there anything more we need from Bitcoin, as a mean of exchange, than faster confirmation time and a privacy option? I don't think so.

Thousands of other sidechains will be created for different purposes but none will be competing with Bitcoin for its position as crypto-money. Some might try, but will realise they will be better off applying their competing idea to an actual altcoin.

Back to my two utility sidechains : these will never take over Bitcoin as they are effectively serving only a certain application or subset of money. Considering these two applications are using a BTC-peg as their transacting unit they are, barring some technicalities, a part of the Bitcoin network and closely tied to the native bitcoin unit, so much in fact that their 1:1 value will always gravitate toward each others and this correlation will become narrower as arbitrage comes into effect and time allows the technology to mature.