... it just so happend that 35mm was the goldilock format for most people striking the balance between size and weight of the body, and image quality and DOF control.
Those factors plus cost maybe? There are some problems with thinking that the same format will be the ideal balance in digital as it was with film:
- image quality at equal format size is far higher for digital than for film, pushing the balance towards smaller formats
- cost at equal format size is far higher for digital than for film, pushing the balance towards smaller formats
- evidence from the market place: almost eight years after the first "full frame revolution" (Kodak 14N, Canon 1Ds), and almost five years after the "second revolution" (Canon 5D), the smaller DSLR formats still outsell 35mm format by well over ten to one, and the 35m format DSLRs still cost far more than most SLR users have ever for a camera.
P. S. on size. It is laughable to suggest to a telephoto enthusiast like me that the 400mm and longer lenses I would want in 35mm format are anywhere near to an ideal balance of size (and price) vs performance. For most SLR users, the bulk of such telephoto lenses was merely the price we had to pay in the bad old days when film offered far inferior combinations of resolution and low light performance than electronic sensors now can.
P. P. S. on DOF control. By the end of the era of 35mm film cameras, the great majority of users of those cameras used them with zoom lenses of maybe f/5.6 at the long end on SLRs, slower on the compacts that accounted for the great majority of 35mm film cameras. Thus the DOF control they had was no more than the smaller mainstream DSLR formats now offer when used with lenses of suitably low minimum f-stops. So the mainstream DSLR formats like EF-S and DX can match or exceed the IQ and DOF control that most people were getting with 35mm film back then.