I believe all of the above are factors, Simon.
IIUC the underlying dependency is the encryption chips for the control board, formerly marketed by MediaTek, but no longer available for quite a while now. Esoteric still (presumably) manufactures mechansisms although they have stopped supplying them on an OEM basis. In a shrinking market OEM volumes would only improve their economies of scale so this is presumably for other reasons, possibly to safeguard their inventory of the above chips? No idea.
Now as you say D&M have followed suit. We have a healthy inventory of the D&M mechs so we are in as good a position as anyone and probably a better position than most.
As to whether any other manufacturer steps forward to fill the void, only time will tell. The two major drivers of economies of scale for production of silver disc mechanisms, the mass market for CDs as the default distribution medium for music and the computer market as the default distribution medium for consumer software both dried up long ago. The audiophile market for specialist trabsports is infinitesimal by comparison.