What would the downside be to the RKBA by having the Second Amendment apply to State and local governments as well as Federal?
None whatsoever. In fact, crime among the states would be so low that spending on law enforcement could be decreased by one half with no noticeable effect to the average citizen. The gungrabbers at the state level don't want to implement this test. They'd much rather the Feds try to form the 'more perfect' union.
'More perfect' is best acheived from the bottom up rather than top-down. Ask any serf.