Phasing them out had to do w/ conflicts being at closer range and the 109 is supposedly a more versatile all around platform. Of course the new 109s (Paladins?) have gps systems in them that do away w/ the need for FDC.
The M109 Paladin looks more like a tank than a howitzer, formidable either way. Now that we've hijacked this thread, one can legitimately state the US military has been dealing with SHTF scenarios since the US Revolutionary War.
I've also heard the reference made to the size of the US hunting population outnumbering that of all military personnel in the world combined. No invasion on US soil, including that of our own military will stand. US troops may be called in to restore order and oversee disaster relief distribution.
But for the most part, if the balloon goes up (there's a Cold War military term for you), you will most likely be on your own, in a small group of like minded individuals, or neighborhood group. How long that situation lasts will depend on how quickly things return to normal. Remember, except for the military, all mission critical personnel that run government, banking, utility, and IT infrastructures live in the same neighborhoods and shop at the same grocery stores as you and I. If there's nothing available to you, then it's pretty much unavailable to anyone.
So, I think order will be restored in short order if at all possible. Longer term scenarios aren't good for the urban centers, and a retreat will be necessary, as well as caches of supplies.
What is the trigger for a SHTF scenario? Impossible to predict, but not impossible to anticipate or prep for the outcome. It's never too late.