Dunno how good your german is
I don´t know 66GB-discs, it don´t exist in the BD XL specs. The 100GB could be BD XL compatible, don´t think they made an alternative standard for 100GB-discs.
I never was the 1st to get a player for new technologies, so I don´t had the compatibilty problems. My first DVD-player I bought 2004, it was cheap and played +R/-R, VCD, SVCD etc. Expensive brands weren´t so compatible.
Atm I have no plans to buy a UHD-player.
AFAIK the developing for successor of UHD had already started, Japan presented a standard 2016, but the name and the standard itself can be different to the UHD-successor.
If all the formats are to expensive (player, discs), I don´t know how successful UHD and it´s successor can be. Maybe the streaming-provider will be rule the movie-market in near future and UHD will only be for the guys who collected movies on discs.
Personally, I don´t like formats let me think that the developing of the copy-protection costs more than all other things.