“only one drive JD really needs - it is 1541.”
Agree, but JD's auto detection of double sided floppies really makes the 1571 a lot more usefull in combination with the C64.
“I don't understand why is 1541 so popular...”
The 1541 is so popular because it was so popular in 80's. It was so popular in the 80's because of economical reasons: Joe Average could own a computer with a real floppy drive. Single sided floppy's were no major space limitation for a lot of C64 software and therefore flipping disks to access the backside was acceptable. The speed limitations were annoying but could be easily circumvented by fastloaders.
It fact, I believe the 1541 was a major contribution to the C64's vast and rich software catalogue. Games on floppy disks could be significantly more complex than games on tapes, this gave the C64 an advantage over many of its competitors.
“I don't want that M65 will be used for C64 mode only... I know that you all will be angry, but look forward and see M65 mode with all enhancements and understand that C64 is not king anymore.”
Well, the advantage of the C65 has over the C128, is that hardware wise, all functionality is available in C64 mode. When the C128 switches to C64 mode, it basically is a C64, very few C128 functions can be accessed from the C64 more. Therefore C64 games could not be enhanced with C128 features, you had to do a full rewrite for C128 mode. With the C65 this is not the case: You can for example use all functionality of the VIC-III in C64 mode.
Of course, software written for C65 mode would be the perfect world, but rewriting software for C65 mode needs significant resources and the question is if the community will have enough of those resources. Enhanced C64 software needs much less resources from the community and could already make the Mega65 a success from a software library point of view.