I hear this argument a lot in the EE community. Yet, the cheapest 8051 part in single quantities on DigiKey: $0.43. Cheapest ARM Cortex-M0 part on DigiKey: $0.47. I'll take the ease of development and features on ARM for $0.05. You can develop without proprietary development environments. I do my development with a basic gcc+make setup.

Of course you can pick up old parts which fell off a truck for cheaper but I'd rather not do that when I depend on these parts for many years to come.