switches to a completely different and unique shield.
This is the same logic that Tim kept going back to with Vermont and why the Vermont state routes were never finished under CHM.
Indeed, which was hugely annoying and a fiasco we don't need to repeat. But in that case it was about whether to include the circle routes at all, which is far more consequential a decision than deciding which system a route or group of routes goes in.
For what it's worth I don't think 407/407 ETR should be split. Seems like unnecessary effort that wouldn't really accomplish anything beyond forcing users who've driven both portions to put an extra line in their list file. But I do acknowledge that valid arguments in favor of doing so do exist.