A permanent fallback appropriation would remove congress’ ability to reallocate or reapportion spending without a 2/3 majority vote in both houses to override the presidential veto, something that seems unlikely in the modern party system.

Also military funding has to be reappropriated regularly under the constitution, a permanent military appropriation is unconstitutional. “To raise and support Armies, but no Appropriation of Money to that Use shall be for a longer Term than two Years;”