As Noel points out, what would have happened is that the territories would have been redistributed between the other surviving reigning branches. The only point of contention might have been the Grand Ducal title which should perhaps go to the male primogeniture line. On the extinction of the entire Ernestine male line either the succession would pass to the nearest female to the last male, or a "Baden" solution mighe be found, by which the nearest morganatic male line is demorganaticised. But this could not be done when not reigning as it would require the act of a reigning sovereign - and the chances of restoration are slim, to say the least. However the Saxe-Meiningen line has a lot of Freiherren von Saafeld, descendants of Ernst, brother of the last reigning Duke. When the latter abdicated, Ernst refused the succession and had already married unequally. Of course there are plenty of Saxe-Coburgs - aside from the senior line headed by the representative of the Duke of Albany, excluded from the SCG succession by a morganatic marriage that was actually to a noble woman (whereas the present Duke of SCG is married to a commoner), there is of course another line, considered dynastic, and which retains the right to SCG should the junior lines become extinct - that is represented by HRH the Duke of Gloucester. The renunciation by Edward, Prince of Wales was made at Windsor Castle on 19 April 1863, and was accepted by the Landtag of the Duchy and then on behalf of the three younger British Princes by their legal guardian, King Leopold of the Belgians, at Laeken Castle a month later. This renunciation retained a contingent right of succession for
himself and his issue in the event of the extinction of the male line descendants of his younger brothers. Therefore, should the male line of Duke Andreas become extinct, then arguably the heir would be whoever is Duke of Gloucester.
The Saxony branches don't really have many eligible male dynasts.
Meiningen only has Friedrich Constantin at 37 still unmarried.
Andreas of Coburg has two sons and one grandson. His daughter-in-law is 43 so unlikely to have more children and his younger son unmarried at 41.
Saxony itself had to go to the Gesaphe branch through a female line.
It is an anomaly among the German princes that the Ernestine branch of the House of Wettin has never fully adopted primogeniture. Instead the branch has allowed several princes to share sovereignty at the same time. Over the years the geographical divisions have varied tremendously. Nobody, I think, regrets that today we do not have dukes of Saxe-Coburg-Eisenach, Saxe-Coburg-Saalfeld, Saxe-Eisenberg, Saxe-Hildburghausen, Saxe-Jena, Saxe-Marksuhl, or Saxe-Romhild. The divisions of land between the princes of the Ernestine branch have always been temporary. There is no reason why the divisions which existed in 1918 must be maintained forever. In fact, it is likely that in the future there will only be one Hauschef for the entire Ernestine branch. I can't see that this is problematic in any way.
Noel S. McFerran