After a successful soft-fork allowing for 2-way pegged assets any entity proposing 1-way pegged assets will be viewed skeptically and economically disadvantaged by the market perception of losing interoperability with the MC. The market will prefer 2-way pegged assets over 1-way pegged every time, and that is a net benefit to the total value proposition of bitcoin surely.
There is no loss of "interoperability" as long as exchanges (centralized or otherwise) exist. Non-pegged alts have floating value, but they aren't unconvertible currencies. Those are two different concepts.
It remains to be seen whether the friction of conversion is preferable to the friction of pegging, but isn't clear in the abstract.