I never liked app engine. To me it seemed like you have to lock yourself into google as a vendor because your application has to be designed for app engine.
Relying on a service hosted by a third party is fundamentally different. Nobody, not even Torvalds or van Rossum, could throw a switch and shut down your Linux boxes or your Python interpreter, so any needed migration to an incompatible system can happen at your convenience.
This argument come up often, but makes no sense as it ignores the fact that software rots. I cannot keep running Python 2.x indefinitely without maintenance because knowledge of its flaws will slowly escalate until it is a security risk to keep operating it. If this doesn't happen to the tool itself, then it will happen to one of its critical dependencies, like an old version of SQLite it requires, or the previous generation of OpenSSL.
Projects like Python, that simply don't care about maintenance costs of things that rely on it are simply being naive: they are pretty much saying "yes, you invested a ton of time into building code for Python 2; but those days are over: we, at some random shifting date in the future, are going to drop support for this, so if you want to get security updates to it, or hell: even upgrade to a newer version of Ubuntu with minimal pain, you now need to drop everything you are doing and reread all of that code to update it for our seemingly arbitrary changes, retesting the whole thing in the hope of finding any regressions you introduce; that, or go off-grid, maintaining this code and related infrastructure yourself". Seriously?
Imagine if Linux 2.6.100 decided to drop support for socket(), because it is (truly) a painful API to support multiple transport layers with; /even if/ they said "dude, you can still just type socket_old, and we have a crazy preprocessor that hopefuly, if your code is obvious enough, does this search and replace for you", would you still be able to take their dedication to being a platform seriously?
There's a big difference between flicking a switch to turn off python 2, and slowly deprecating it over a period of years to a new (better) system with well doucmented differences and a clear migration path.
Python 3 started in 2006 (http://www.python.org/dev/peps/pep-3000/), if you've managed to stave off software rot for the last 5 years, I'm sure you can take the extra step to support python 3.
None of those are examples of vendor lock-in. Linux, sql, and python all come from many different vendors. If you were using, say, Debian, SQLite, and IronPython, and they all exploded, you could switch to alternatives pretty easily, I think.
I never liked relational databases. You have to lock yourself into SQL because your application has to be designed for SQL.
I never liked python. You have to lock yourself into python because your application has to be designed for python.