I would think that in mass manufactured products AVRs are actually more common than Atmel made '51s, and for the whole '51 market Atmel seems to be mostly irrel

I would think that in mass manufactured products AVRs are actually more common than Atmel made '51s, and for the whole '51 market Atmel seems to be mostly irrelevant as most '51s you see today are various highly integrated special purpose SoC's from mostly asian manufacturers. I've not seen anything that can be called "consumer electronics" that contained Atmel made '51, while I've seen some such things with AVR's (probably most often with mega169). In various low-volume industrial stuff there are heaps of Atmel made '51s, but only reason for that is familiarity (and to lesser extent AVR-compatible ISP). Another thing is that Atmel is prone to supply chain problems. In our case it meant that we had redesigned the same device three times, first design was based on mega8, which in late 2010 we found was completely unavailable in any non-prototype quantities, thus we found that our distributor had cheap inventory of AT89C2051 and went with that (with all the development pains that are inherent in non-ISP '51) and on last major revision we went with MSP430 (G2253 IIRC) and didn't have to change anything due to supply problems for 3 years. In any case I believe that for many uses of Atmel microcontrollers the familiarity and avaiability (like mega8 being fifth of the price of PIC16F84 in unit quantities in ~2007) were the major selling points.