I don't think being locked down to a service is that important in this case. I see Anvil as a potentially valuable tool to create prototypes, to quickly iterate on an idea... then when it is validated, you can build it with more conventional (and safer in terms of future) tools.
Yes, its funny but I think of python itself as an exceptional good "prototyping" language. Thats not to say it can't be used in production, but I think the real power of python comes from its ability to go from 0-100 in a very short amount of time (powered by all the great python libs).
So in a sense, Anvil is valuable at its price point for just as a web prototyping service. Most new developers obsess on things like "scalability" and "MVC separation", etc and in 95% of cases their creations never ever generate enough traffic to warrant any of that. I see Anvil as a tool that would allow developers to sketch out an idea, quickly iterate features based on feedback, and when warranted, move to a full production stack.
