He's already pointe out Germany wouldn't be able to actually execute the subsequent plans for quite some time, dye to logistical reasons etc.
Therefore it had to be decisive.
He's stated it based off of a historian who did not have access to the original German documents; my position is based on the army officer who rediscovered copies of the documents in US archives and wrote a military analysis about the offensives. Stone is a good historian for the Eastern Front, but he is not a specialist on the German army in the west, nor did he have access to the general staff documents that Zabecki did.
The problem is two fold, from one part the Germans had low supply and morale was not great, even if Micheal was succesfull and the logistic better planned, continuing other offensive will be very hard (and can start a series of mutiny) and second once the Battle of Vittorio Veneto start and A-H is knocked out of the war, Germany is surrounded and her south open for an invasion.
Morale is at a peak prior to the offensives and the supply situation for the offensives is actually quite good, despite the home front situation. You do know that after Michael IOTL there were 3 more major offensives, right?
Their failure to knock out the Allies was due to Ludendorff botching the plans, not due to supply issues or lack of reserves. They were misaimed and frittered away their energy against pointless objectives. Micheal itself doesn't need to be continued, offensives elsewhere on the Western Front would follow on as per OTL.
The collapse of Austria wasn't until the end of the year and the Allies were in no position to launch an offensive via Austrian territory even in 1919 if solely due to the rail situation.
So once all that become clear, Germany remaining options are negotiate from a stronger position while selling it's allies and throw them to the wolf to survive.
There really isn't a need to sell out any allies (other than perhaps the Ottomans to a degree, which is simply acknowledging the reality on the ground in the middle east) if the Spring Offensives in France go well. France cutting a separate deal effectively ends the war and Italy won't be on Austrian territory by then, nor will the breakout from Salonika be possible by then either, especially with France out.