I think this is a very bad idea to be able to change the default strategy for many reasons:

* First and foremost, using `render()` is a way to render a "partial" that needs some logic associated with it (think `include` vs `render`). Then, you might wat to use another strategy like ESI or hinclude.

* Because we have many bundles in Symfony (like in Drupal with modules) and they should all be able to rely on something stable. I try to limit the number of moving things (less options, less settings, more predictability).

* Each rendering strategy has its own set of options (like `alt` for ESI or `default` for hinclude), so the strategy should be chosen carefully by the developer and configured properly. If you can change the strategy globally, that could break things easily (and no, we won't add yet another setting to be able to set the default strategy global settings).

* Lastly, I don't think it makes sense to use the ESI strategy for all the render calls (and the same goes for hinclude too). So, I fail to see the real-world use case.

And if you really want to change the default strategy or if you want to implement your own default strategy, just create a strategy with the `default` name and register it in place of the standard one.