All of OpenTable's features are core competencies of a restaurant. A restaurant
must do CRM, it
must maintain analytics about its own performance, it
must be able to manage tables and keep track of payments, and unless you're a no-reservations place, it
must have a system in place to handle reservations.
If you don't cover all of the above bases as a competitor, you're leaving your customers to cobble these pieces together themselves. Maximum freedom? Sure, but how many takers do you think you'll have?
"Hey Mr. Restaurant Owner, I understand you have a substandard but perfectly functional system that runs every major concern of your restaurant. Your employees kind of hate it, but they're extremely proficient at using it since it's the industry standard. Boy do I have a great idea for you! You should throw this system away so you can assemble multiple pieces of software from multiple vendors together to do the same thing, in a field where interoperability isn't even in the vocabulary! How your payments system talks to your CRM system? Fuck knows. But you should do this!"
OpenTable isn't completely immune from competition, but they're pretty much as close as any software company can get. There's a reason why every potential competitor to OpenTable has taken the "full restaurant management system" angle.
OpenTable is vulnerable if someone can execute on the kitchen-sink restaurant management system better and more cheaply. I don't think it's vulnerable to piecemeal replacement.