Its not that a sidechain displaces bitcoin hypothetically and that this is bad; a sidechain is bitcoin, its the mechanism to internetwork bitcoin, to build on it.  Sidechains no more displace bitcoin than HTTP displaces the TCP and IP protocol it is transported on.  Alt-coins and alt-shares are in nominal competition with bitcoin, though it seems highly unlikely they would catchup with bitcoin's network effect nor reach an appreciable real-life usage; but sidechains are not in competition.

Bitcoin has one advantage over sidechains - it has the subsidy, and full node security, so I'm sure it'll be able to defend itself against abandonment or insecurity, and sidechains depend on bitcoin anyway so all bitcoin users on which ever chains have a meta-incentive to see bitcoin main remain secure.   We have decades of subsidy ahead to deal with fee-only security for bitcoin, and sidechains may move forward the ways to do that because the sidechain by default has only fee security from the start.

Anyway one potential use for a sidechain is to host a beta for a major new bitcoin version.  If that version after say $1b value running in it for a year, gets upgraded into bitcoin, that hasnt displaced bitcoin, its facilitated its feature and performance upgrade, which is a good thing for everyone.

The exciting thing about the internet wasnt just the ability to send IP packets, but all the applications and permissionless innovation that could be built using that transport.  Same for bitcoin.