What would you bet against? Its current and/or probable IPO valuation? That it'll be around in 10 years? Because I'd be willing to make a bet (say 1k) for Uber

What would you bet against? Its current and/or probable IPO valuation? That it'll be around in 10 years? Because I'd be willing to make a bet (say 1k) for Uber, depending on the terms. I'd also make a bet for Lyft, depending on the terms.