Sorry, but I don't like this approach. The registerplugin methods are the "old" way to register plugins. I'm in favor of creating new Extensions.

Wouldn't it be easier to switch the default loading order of the Extensions? It is now: core, default, BC. We could change it to core, BC, default.

Thanks for your quick reply.

You're right, it would have been be easier to switch the default loading order of the Extensions : Core, BC, then Default.
But it does not handle the case where we want to register a "Smarty::PLUGIN_MODIFIER" (not a compiler one), wich has an equivalent in DefaultExtension->getModifierCompiler(). For example :

//This modifier will be silently ignored, even if DefaultExtension is set at the end.
$smarty->registerPlugin(Smarty::PLUGIN_MODIFIER,"json_encode",[$this,"myJsonEncodeModifier"])

In that case DefaultExtension->getModifierCompiler() is going to take the lead anyway.

Side question :
I understand you consider "registerPlugin" method as old way to register Plugins.
Does this mean that it could be deprecated in a near future ?