I had both Mimby and Bimby sitting at my desk for some time before I decided to sell Bimby. I don't think the price difference is justified between these two units. The cost was not an issue - it is just that to my ears Bimby is not an upgrade over Mimby - it is more of a side move.
In regards to its upgradability. Look at the original Bifrost owners who bought original Bifrost couple of years ago at $499, upgraded it later to USB 2 ($100) and UBER ($150). And later to Multibit for $250. Shipping for these two upgrades was probably $100 (CONUS). Basically their cost to date for Bifrost Multibit $1,100. When they sell it now even at $500 it is $600 loss (cost of use) over time.
Now take this $1,100 less Mimby of $250. There is $850 they could've used over the three years to spend on dacs (non-upgradable DS like Modi uber, Modi 2 uber - they still could've sold those for 50% of their original cost) with almost the same SQ and will still have he cash left.
I think the only thing that will save Bimby now is a balanced out to Jot, which looks like an expensive option if at all possible.
I'm still wondering how many Bimbys schiit sold after Mimbys release. Probably not many. These are just my $0.02.