Whatever the pros and cons of 40mp it seems clear from recent pronouncements from Leica (Peter Karbe) that they expect the MP count in future models to continue to increase .
This is one of the reasons why they have broken free of the design constraints of the M lenses and incorporated future proofing into the SL and S lenses.
As MP counts continue to increase industry wide over time, and they obviously will, Leica will continue to follow suit with the M, even if, as now, as ever, being about 4 years behind Sony sensor tech. That’s a marketing and sales department decision and, since MP counts are one of the few things left that you can attract buyers of digital cameras with, a bigger number always being better than a smaller number in the public’s mind whether it be MP counts, horsepower, or bra size, then it’s a way forward they probably have to take whether it makes any sense in a rangefinder focusing body or not. Some M lenses are up to the demands of a 40 MP or higher sensor, but the current inherent accuracy limitations with rangefinder focusing make getting perfectly focused results at the 40 MP level with each M lens in your stable a matter of accident rather than skill. That’s not to say that you wouldn’t occasionally get a higher acuity result with the 40MP M than the 24MP version, or that you couldn’t get superb results from either of them.
But, even Leica don’t and won’t make the claim that 40 MP resolution is as shot to shot, lens to lens, fully usable as it is on an SL or S. You’ll get 40MP of something all right, and you can blow that something up bigger, but what that something is will depend to some extent on whether you bumped the camera recently or if it has had its infamous Leica “recent CLA”. Etc.
Yours seems like the more reasonable approach, as those SL and S cameras are not limited by rangefinder focusing, (this is no accident) a focusing method which made the M3 much loved for quick focusing and reportage in the early film days, and still makes for a nice small package these days. But.
The future proofing you allude which the SL and S are equipped to meet, to is the future of higher MP counts and the pressure those put on camera body and lens design.