I totally agree with your thoughts. However, I think the solution will be simpler than you think: Through Ironpython, Python will be able to leverage all the power of ASP.NET running on the .NET framework and Mono.
What can be better than that?
.NET is very, very similar to Java (the platform, not the language). We're talking about wanting very high-level web frameworks, and I know that the closest you get on Java is RIFE. I don't know about .NET.
The trouble is that frameworks for .NET and the JVM are designed primarily for statically typed languages that don't offer cool features like metaclasses. I've used Hibernate extensively, and I can safely say that SQLObject (and ActiveRecord, I'm sure) are much easier, because the language offers features to support that kind of thing.
So, while it may be nice to be able to call .NET and Java code, that is not really a big win in terms of web development productivity.