"It is not possible to be competitive with ones designs by only using one type of processor."

I think there are plenty on this list who can prove otherwise!
Especially in the case of the 8051, where the one family covers such a huge range of variants - and is embedded as a "core" into many other products.

You just don't bid on the jobs which are obviously not suited to an 8051.

Of course, it helps to have a "network" so you can pass them onto others with more appropriate processor skills, and vice-versa