From the Job opportunities point of view, is learning the 8051 alone a good strategy?
It depends, if you like working in big companies, it is not. The word 'embedded' apply to two very different situations:
1) a cellphone, an ethernet router etc is considered embedded and programmed by a team of maybe 50. If that is what you want, start with the ARM or the x86.
2) controls for such as industrial machines, washers, microvawes etc are most often developed by one or two persons. If that is what you want, start with the '51.
Whether you decide on PIC, Moto or the '51 some doors will open and some will close.

Erik