As much as I am a member of the Disneyland Preservation Society, I like the idea. It doesn't detract or mess up Main Street, will help alleviate the logjam when the fireworks are in progress, and if Al is correct, will give some interesting theming possibilities with the whole back alley of Main Street thing.
“No worries, stay calm, one question. Do you happen to know how to fly this thing?”
If they do expand main street then the parade will end backstage of the fire house and city hall.
It's impossible to reroute the parade to the firehouse because of the Jungle Cruise taking up the space behind it, so the ideal place for a back alley would be on the left-hand side to open a new pathway for those trying to avoid the crowds during a parade.
I don't think being a Disney Preservationist, and this idea, are contradictory at all, especially if the theming is carried out as they seem to be planning. This idea is non-intrusive to the historic fabric of Disneyland, and is both practical and thematically correct. Basically a win-win it seems to me.
If they are able to do a really good theming and make it look like a back alleyway then I don't think there is any interference in preservation. They are not taking away something old and beloved to us and replacing it. They are giving us something new to help with traffic and give a different view of Main Street