As you say, there is no effective way for them to lock users in, but they don't need to. They can outspend and outlast Lyft to make them run out of cash and/or devalued to the point where they can acquire them and shut them down.
At that point, any (future) new entrant to the market will face a steeper uphill battle than Lyft did. In the process, they may lose some customers to real cars or public transit, but they'll remain the only player in this space.
No they won't face a steeper uphill battle. They'll have a much easier time: legislation will be clearer, drivers will already be driving for uber so installing another app is trivial, riders will already be familiar with app-hailed rides and so installing another app is trivial.
These are good points, but at the end of the day, a provider is going to have to pay out to drivers (and collect fares). Uber will be difficult to compete with on price for the exact same reasons that Walmart and Amazon are difficult to undercut consistently.
At that point, any (future) new entrant to the market will face a steeper uphill battle than Lyft did. In the process, they may lose some customers to real cars or public transit, but they'll remain the only player in this space.