From what I understand and please do correct me if I’m wrong, at least 50% of the hashrate has to opt-in to support a drivechain, and all users that are interested in the drivechain need to run a client that supports it. If a very significant portion of the economic activity of the Bitcoin ecosystem moves to a drivechain, wallets will be pushed to adding support for it. It seems to me that activating drivechains would very likely significantly increase complexity of Bitcoin software in the medium-term.